Chris is spot on.
Shooting in water adds all sorts of complications which impact on the overall impression created. Whilst non (?few) of us want to create the image in post-production, it does have a critical element especially, I'd argue, in wide-angle shots where increases in things like Clarity, Dehaze and Vibrance can have a huge impact on the crispness, punch and wow-factor. MP is not the issue.
As I wrote earlier, for many years I used a 24MP D300 and sold A3 printed images that were really sharp and could, I'm sure, have gone to A2. You need a good RAW file for sure but also good post-processing.
As Chris suggests, and you may already be doing, shoot RAW, make adjustments to create the style of image you like, re-size to whatever you need for output - then take a look at the critical sharpness issues.