DARN!!!!! The Brit beat me to it

I can't help you on everything but I can help on the BEC question:
- NO, you don't need a receiver battery pack. The BEC gives a regulated output to the receiver so a separate pack isn't needed.
- I don't know which ESC you're looking at specifically but the BEC output wires, to the receiver, should have a standard connector that fits receivers
Could you give us a brand and model number so we can look at it? That would help considerably.
You said you're looking at building a Dumas commuter boat? Which kit are you looking at?