Sidemount on a bouncing, pitching boat is a sight to behold. And not that attractive a sight.
Now you axed about small doubles. Your options are 1) twinned AL 50's; twinned LP 45/50's; twinned LP66's; or twinned LP85's; maybe AL80's.
I don't know how big you are or what your air efficiency is. I use HP120's, and I've been on boats where women with LP66's have matched me for air consumption. Now part of that is that I'm about the size of a small NFL linebacker plus there's the thing about men's brains... my brain in particular... requiring more oxygen than women's brains... but I don't want to open that can o'worms.
I always thought that the twinned LP45's had a nice balance to them. They're not the squatty body profile of the LP66's. The LP 85's twinned will be kinda awkward and damn heavy climbing up a boat ladder. I have a nice old set of the old Fabers twinned that I only used a handful of times since they were a pain to use on a boat. One of these days, I'll get rid of them. One of these days. They sit next to my concrete mixer and hydraulic tubing bender which I also do not use.
The HP130's used to be really popular down here in Floriduh. Damn those things were heavy. I used to load them for lesser men at Fill Express since they could barely lift them in the shop. A single tank, righteously and judiciously overpumped, is a true Blessing Upon Mankind, but getting kinda tough to find.
Good luck with your choices. There's always the rebreather option. There are some nice units out there. Tap into some of that loose Duke money floating around up there for that and the matching scooter.