when installing batteries in a boat, do you have to ground them to the frame of the boat just like you would in a car or do you just hook up accessories positive to positive  and groung to ground.

You don't ground to the boat hull, no.  

First off, unless the boat is aluminum, there is nothing to ground to.  Even on an aluminum, you don't want any connections of any kind (negative or positive) going to the hull, this can create all kinds of corrosion problems.  Everyelectrical connection should have a positive and a negative wire going to it and everything should be isolated from the hull.

thanks guys that was what i was thinking but wanted to make sure. thanks again