The Zara Spook is a great example of a topwater fishing lure

the lure pretty near the entire time it is away from the boat.  Sure there are some that dive a bit before coming back to the surface, but the majority of the time they will be on the surface.  On top of that nearly all of the stikes will happen when the bait is on the surface, which is what makes this type of fishing so great.  Now, topwater baits are generally not meant to be walleye fishing lures.  No, you are looking for aggressive fish that aren't afraid to rise to the top to hammer a lure.  Generally we are talking about bass, northern pike and the like which are known to be aggressive at times and willing to boil the surface of the water to find a meal.

So what are the general ways to fish topwater fishing lures?  Well that really depends on the bait.  There are some topwater lures that needs to be fished fast.  I'm thinking of lures that will sink if you don't reel them in quickly.  A good example of this would be a buzzbait.  A buzzbait is kind of like a spinnerbait that is designed to stay on the surface.  Blades twirl in a circle and cause a ruckus on the surface designed to get the fishes attention.  But generally, the lures that I have had the most luck fishing on the surface are slow movers.  Often a good rule of thumb is to cast the lure to a likely spot and let it sit there until the ripples dissipate.  This can be hard to do, but often pays off in spades.  As soon as it hits the water, trust me the fish will notice.  If you move it right away the fish may be spooked.  By leaving it sit they get a chance to come up close and check it out.  Often from here it is best to just twitch the lure with your rod tip and move it a few inches at a time in a haphazard fashion.  Don't get me wrong, there are times when the fish wants it to really be moving, but it seems more likely they want a sporadic retrieve.  Twitch then wait.  Rinse repeat.  Another option is to "walk the dog" as they say, and basically twitch the rod back and forth.  The intent is to get the lure to "walk" back to the boat atop the water.  This can be very effective in the right conditions.

The last thing I will say about top water fishing lures is that care needs to be taken as to when to set the hook.  Make sure to count to 2-5 before setting the hook.  You want to be sure to let the fish get the lure good and in his mouth.  if you set the hook when you see them hit it you will miss about 95% of the fish that hit your presentation.  Often they are just striking the bait for effect and will then hit it again and put it in their mouth after the initial hit.  This is imperative to making sure to get more fish in the boat while fishing with top water baits.

Some classic brands and examples of topwater lures include the Heddon Zara Spook, the classic Hula Popper, a buzzbait, and many floating crankbaits such as the Rapala original floating lure (although to be truthful I prefer the Rapala Jointed Minnow, also called a broken tail minnow).  If you want a look that most fisherman don't use consider going with a Japanese fishing lure or a custom painted crankbait.  Both are great ways to give the fish something they have not seen before.

PR: wait... I: wait... L: wait... LD: wait...

That said, if you are fishing shallow water filled with submurged stumps / wood or heavy vegetation in places then I would beef up the line you are using.  In shallow water losing dive depth can actually be a very good thing and in addition should there be any places that the fish would swim into that would be hard to pull them out of I have some extra umpfh to heave them towards the boat.  Similarly, if I am trolling a crankbait in rock laden water I would also beef up my line.  When I say beef up line, I mean move up to 14-17 pound test line for these applications.

So, how much does line diameter affect the dive depth of a crankbait?  Well, after doing some research it seems to me that the right answer is something in the neighborhood of 10% per 2 lb test.  So if you are trolling a size 7 Rapala Shad Rap behind your boat, it would be expected to dive to a maximum of 11 feet with 10 pound test line.  So if 8 pound test were used instead, it would be expected to dive to 12 feet.  Likewise, if 12 pound test were used the maximum depth would be expected would be 10 feet.

What do you think is the best line for crankbaits?

When working a crankbait for smallmouth, cast out and reel straight in.  Smallies typically don’t like erratic retrieves.  This goes for zara spooks and Senko retrieves as well.  If you see the smallies chasing the bait, but not eating it, switch to a different color crankbait. 

To walk the dog, cast the spook out as far as possible.  This method works best when using a baitcasting rod/reel with a rod that is between 6 to 6’6” long.  Cast the spook out as far as possible.  Let the bait settle on the water until the ring disappears.  Many times bass will hit the bait before you start.  When ready, have some slack in the line and make a short jerk down on the rod tip.  Reel in some slack.  Perform this in rhythm, so you jerk the rod tip down while reeling in some slack.  Continue to do this all the way into the boat. 

If you get a bite, let it wait for about 3 seconds.  With every topwater, it’s best to wait before setting the hook.  This will increase your chance of catching the bass.  After waiting 3 seconds, count it if needed, set the hook.  If the bass made a swirl at your bait, but failed to even eat your bait, reel in fast.  Set that rod down, if you have a 2^nd rod ready with a Senko, cast it out to the exact spot you had the bite.  Let it sit.  This is called a follow up bait.  Follow up baits are great for top water.

The Rapala DT Series is a great example of a search crankbait

The key to using a crankbait as search bait is to have your GPS or buoy ready.  If you get a bit, mark that spot.  There was a reason that fish bite you there.  Chances are that more are in vicinity, if not on the same spot.  Cast all around after catching a fish and note any differences.  Maybe there is an underwater point, rocky hump, inside weedline, or so on.  All these items can give you a pattern that the bass are on.  Take what you learned from that first catch and use it to your advantage at the next spot.  Find another location that has the same structure.  You should be able to reproduce the same success in other locations.

 Two of my favorite crankbaits to use are the Rapala DT 16 and the Rebel Wee-R.  The Rapala DT Series allows me to cover a vast amount of deep water.  Pick this lure when your hitting weedlines or edge of weedlines in 10 to 12 feet.  Anything shallower and you will hang weeds up way to often.  The Rebel Wee-R crankbait is great for fishing over shallower water, in the 8 ft or less range.