It never goes below 50% but does get discharged with starting and the windlass down a bit, I'd guess 10 to 15%.

I use an automotive charger in the fall and fully charge(but this is not equalizing), and leave them on the boat until spring. There's nothing to gain by taking them off or running a trickle charger. The self discharge over 5 or 6 months doesn't hurt them.

I buy just standard lead acid deep cycles, nothing high grade. That's why I'm curious if more expensive batteries may be a better value in my case.

With the house batteries, they can often be deeply discharged, not fully charged as we're never plugged in during the season. But the starting battery, that seems like a pretty good test of the lifespan of a deep cycle. I noticed last week it was slightly below the house after a full charge and rest.

Again, it's still "good", but it's not as good as it was new. I doubt any of the deep cell types deliver 100% throughout their life span but some must be better than others.

I hear many get a similar life span that I get with my batteries and usage, about 5+ years. After that point, I'll notice a large difference in resting voltage after a full charge. Some are still "good", but I retire them.