Yes, we've had water but I am not sure we've done floating devices. Let me look...
Personally I like themes which is are a challange to make look good on a gameboard.
Here's a list which is passed on since last years BSC:
Speed
"HISTORICAL ACCURACY TO SCALE."
WWI
Peace
Animal
Vegetable
Mineral
Earth
Poisonous
Spotted
Production
Noise
Wreck, Broken
Exit
Small
Pringles tube
Computer fan
Box of matches
Trees
Generic terrain
Plant life, VegetationDifficult terrain, broken ground, obstructions
Bunkers
Warmachines.