Six-Liner
Six-Train
Raiden
18 trains straight away.

Can anyone name 18 boat Transformers? Not sure I could.

 

You're right about those combiners. Six-Liner and Six-Train are kind of expensive now and I don't really like the small alt-modes.

As for boat Transformers, there are at least 4 or 5 of them in G1. Seaspray, Broadside, and Quickswitch come to mind. There are several boat Transformers that come after G1, but I don't remember their names.

 

Tidal Wave, Shortround, Thunderblast, High Tide, Storm Surge, Hearts of Steel Shockwave, Riptide, Cybertron Mirage, Nautica, Undertow, Seawave, Deep Dive, Submarauder, and Waverider.

 

also the Sea Attack Minicon team and their fucktillion repaints.
also, its not a Transformer per se, but i'll take any opportunity to mention Webdiver Galleon. the Giant Robot Dragon Pirate who turns into a Pirate Ship.

 

You're only hurting your case, though, because Gradion and Phoenikon are both trains...  

edited to add: oh wait, Sharkon was also a boat, so it's a wash.