Does not sound like a power supply problem unless the unit is actually switching on / off intermittantly. Is the unit obscured by anything metal? i.e. have you got it mounted under a dash for example? Other than that the unit may be faulty, you could remove it from the boat and connect it to a powder supply in the open air and see if it improves or take it back to the retailer and ask them to compare it to another one in stock. I was in a tinny with one of these on Saturday and it did not have any of the problems you describe, in fact it worked really well.
Sometimes when you first fire up a brand new unit up they can take a while to get their bearings but then to lose the signal constantly is a either fault or internal antenna being blocked out. Another option is to try a full re-set but save any marks you have entered onto a card first.