If you're looking to top off the battery just prior to splashing the boat, just about any automotive charger will do. In this case all you need is enough charge to start the motor, then that will charge the battery.
If you're planning on leaving the charger connected while the RIB is on the trailer, then you want to select a trickle charger. A regular car battery charger will likely cook the battery if left on for long periods of time.
Ideally, a long term charger will be a three stage device: bulk (high voltage, fairly high current), top off (high voltage, fairly low current), and maintenance (low voltage, and very low current; and, ideally, something that monitors the battery voltage and comes on only when the voltage drops.)
For long idle periods, I use a little 500mA float charger, though I start with a topped up battery. It would probably take a month or two to charge a depleted battery. Can't seem to find a link for the device now (maybe not made anymore?)
Apparently West Marine (aka Worst Marine over here) still sells them:
At 500 mA peak current, it is unlikely to overcharge even if the auto-shutoff thingie fails.
Hope this helps;