Greg1:
Here is a link that may steer you in the directions to influence your decisions on battery selection and perhaps more importantly your charging configuration.
Jerry knows: the Deep cycle battery is designed to go deeper into a discharge status than a starting battery.
Look also at your reserve capacity ratings.
http://www.batterystuff.com/tutorial_battery.html
After you have read to the Dos and Don'ts of battery maintenance... you may likely come to the conclusion that this/these batteries should be hooked up to your vessels charging system, rather than waiting for a later time to recharge them. Even your satellite battery for a 24 VDC series connection should be rigged in such a way that you can charge the battery when the Electric reel is not in use. (I think that is what the Isolator that Islandshark mentioned, can do for you.)
So even if you use the battery(s) to a deep state of discharge, it will not be long before it is coming back up again to a full charge. (A Do.)