Macfern, I can not tell what size outboard you have (another reason to update your profile with "boat-type"), but like I've mentioned before, outboards typically have smaller charging outputs than autos. Any hp between 200-250 (4-stroke Yamaha) will have 45-46 amp charging output, 35A for a 150 hp, and 25 amp for a 115. Regardless, I would bring a little charger to utilize whenever you can back at the dock. If you're really worried about it, be sure to charge both batteries on the way out but "reserve" one of them for last-ditch-effort starting. After all, all you need from the backup battery is one good start. Just be sure to prove that one-time starting ability at the dock first. If you feel both batteries have been charged equally, take a voltmeter reading from the isolated batteries. Reserve the one with the highest reading as the never-used backup battery. As a last resort, combine the two for starting, as a questionable battery can detract from the cranking capacity of a good one.
You may even consider a stowable solar panel with a charge controller and just plug it in to a cigarette/power outlet for easy connection, in order to keep your batties topped off if the dock has no power. From what I've heard, and without knowing all the particulars, my gut feeling is that your 2-year old batteries are probably up to the trip you have planned. If you feel they're not once you get down there, batteries are sold everywhere and quick to install.
I forgot to mention in the prior post, whenever trying a new float-type charger, verify the charger's regulation of 13.8 volts by taking a volt meter reading every so many hours. Insure that once the voltage has finally finished climbing, that it does not exceed 13.8 volts.
What are the particulars of your trip? Are you trailering, slipping? Power, no power at the dock. How often do you plan to use the boat on the trip?
Wish it were me