RE: shallow water
If i would reply to this topic before him, i would say exactly what o2bnme said. The weed fins don't go as deep in the water as the regular fins, but they have just as much SURFACE, and that is what really matters when trying to avoid spin-outs. Don't use a 32 cm fin unless you are really experienced, lighter than 70 kg and sailing really small sails (<5.8). But if that is so, i can't see why you would be using a 155 litre board.
All the best