I have a Trailstar trailer under my Crestliner CMV 1850...near the end of last season I started showing rapid wear on the passenger side tire (starboard of the boat) on the inside only.

I assumed it was a tire problem and was going to replace them anyway.

I swapped out to the spare...and am noticing the same uneven wear already on the new tire.

I pulled it all over for two seasons prior to seeing this problem.

I'm thinking I have a slightly bent axle, but am curious if it can be anything else?

Your best bet is to see if you can find someone that has the ability and know how to bend and align straight axles like in the early 50's etc.  They should be able to set the cast and camber.  I have all my boat axles aligned for a 13 1/2" tongue height, that's one of the very first things I do when getting a boat.  Never have seen one that didn't need it.  Once you get it aligned, make sure you use the same tongue height every time because that changes the caster and camber if you are higher or lower than it's set for. This makes a huge difference in good tire wear and how it tows.

Actually, some of the old heads that know what they are doing will recommend letting them weld reinforcements on it to help keep it in alignment.