If you have mahogany planking, it's probably light red or dark red Philippine, right?
Sapele is most comparable to dark red Philippine Mahogany (Meranti) with almost the same numbers in terms of hardness, strength, etc. and is considered more durable (rot resistant). It's shrinkage characteristics are almost identical also which might help if you winter-store on the hard.
Compared to light red Philippine, it's as much as 40% heavier, which probably isn't an issue if you're only making repairs. I'd recommend using quarter sawn stock as it is more stable than flat sawn, especially considering moisture changes in a wood boat. It's a little harder to plane and fair as its interlocking grain can result in tear-outs more easily than the merantis but under paint, that wouldn't be a real problem either.
My first choice would be to make repairs with the same species as the boat is built with but if you have a reason to use Sapele, and you have good stock available, it should work fine.