starting problems

by jon
(richmond va)

I have a 1973 18 ft pen yann boat with a 350 inboard engine. When I first start the boat it runs perfectly and if I just ride all day it does great. However, after I ride for about an hour I like to sit and fish and often the wakes from other boats rocks the boat back and forth. When I go to start it I have to give it some throttle and it does start but wont idle. I try to drop it in gear to go and it shuts off. I have to rev the engine and quickly shove it in gear to get it to take off. After it takes off it seems as if its only running on 6 cylinders and then it will start running normal after you get the boat on plane. It has a carb and takes fuel perfectly until it sits in the waves. Also, when I start it after its been in the waves it wont rev over 3,000rpm's. It starts cutting out. I use 93 octane gas from regular ststion but add the marine stuff to couteract the ethanol. The gas tank is only 5 years old. Could it be that it needs a water seperator?

Thanks for the assistance. Jon