As long as your boats total electrical load never exceeds 3 amps it shouldn't be a problem. However, if you use regular (incandescent, xenon, etc.) bulbs, and you have a fairly powerful stereo, you could easily exceed 3 amps. Also, a fixed-mount VHF will likely draw 6+ amps while transmitting.
The other thing to consider is that your trickle-charger might not have been engineered for long-term continuous use.