If you really want to sail hard, check in with the local sail clubs (Dunedin, Gulfport, Davis Island, TBCS, etc) and see if anyone there is sailing while you're down there.

If I recall, the H-18 has daggarboards and the H-16 doesn't? It may have wings, too.

I can't speak for this specific rental operation, but generally speaking the equipment isn't "race ready" and they'd probably frown upon you pushing the H-18 to its limit. The settings (mast rake, jib blocks, rudder rake) will be all wrong, too. And they likely won't want you fussing with it.

If the operator/owner is a sailor, you'll see the equipment is in better shape (like the Islander).

I rented an H-16 once in Key West and there were no trapeze lines (on purpose), and the hiking strap (if you can call it that) was far from adequate.

In many cases, a flying hull would indicate the renter has no clue what's going on and it's probably going to necessitate the rental operator to go out there and get you.

The Wave, on the other hand, can be quite fun in and of itself, even if you can't really get the hull up high. If the wind is up it will scoot quite nicely (again, sails and rigging probably won't be top notch) and you can practice your beach landings at full speed.

I got chased down by the operator (again in Key West) on a Jet Ski when I got their Wave flying a hull (4 people on board, three had to sit on the low side and it was blowing 18) who thought I was going to break something

Heck, I've seen them tow the Waves to the surf line by tying a rope to a 4-wheeler and dragging the thing (makes anyone who sails a high-performance cat freak out)

Longboat isn't known for huge surf, but the chop can be steep and nasty on a NW blow.

And 69 F water temp is pretty cold but tolerable if the sun is out and the air temp is around 80 F or so. If the weather is crappy, the operator may not even be around that day.

Not sure if any of that helps, but I hope you have a great trip down to FL.