Do the rules specify a certain stroke? Is it possible to use, at least at some points, the breaststroke or backstroke?
I'm sure you've heard it before; if you're a sinker, as long as you sink evenly, you won't create more drag. Lack of air, though, can be quite a drag. I heard that wetsuits can add a bit of buoyancy. But maybe adjusting your body role or timing could help.
I'll leave the details to the experts.