Sounds like you've made a decision, but to be honest, theres not any info that would make answering your original question possible. Im sure a 105A alt will be fine, but the decision needs to take into account the average continuous of your stereo and other loads, the total battery Ah and hoe you use the boat/stereo. Just keep in mind though, even a 105A alt is not going to do well to recharge depleted batteries. Asking an alt to recharge batteries should not be confused with an alternator that "doesnt keep up".
Michael
Mikes Liquid Audio