Re: deep cycle battery?
I disagree strongly with Richard above. even for a smaller boat, two batteries are preferred if you have a trolling motor, and especially if you have anything else (Radio, extra lighting, fish-finder). The only exception to that would be if you can pull start your motor. Being as yours is an 80HP, I suspect that is not the case. Otherwise, I use a starting battery for marine functions (this assumes that your motor has the capability to charge the starting battery, otherwise use a separate deep cycle for that also.) Otherwise, you will find yourself in a no-start situation.
As far as which deep cycle, "How much are you willing to spend?" First choice is to get an AGM battery for longest battery life (in terms of how many charge discharge cycles, how well it holds a charge, and how well it tollerates the vibration of a boat.) Get the biggest one that you are willing to deal with weight-wise and dollar-wise. If you can go for the AGM, at least get a deep cycle. If you get the AGM battery then get an AGM charger too. I probably would not bother with getting an AGM for the starting battery, but that is just my opinion.
(Disclaimer: I do work for a company that manufactures AGM batteries and others as well, but I don't think that has had any effect of my opinion. Because I work for them I got an incredible price on my AGM, and I would probably not have went for an AGM if I had to pay retail, but I am cheap. Having bought one the only negative that I see so far is the weight.)
TerryMSU