Ships potable water isn't under sea pressure, so....
Reducing total number of active cases reduces the number of variants. That's a fact.
You're talking about what ifs. It's good to ask those questions and for us to try and answer them, but we know uncontrolled community spread creates variants. We can address that right now. We certainly need to determine if vaccinated people that get COVID-19 have a higher rate of the virus mutating, and they are already studying it, but to worry about the possibly of immunized people causing a variant, while ignoring the reality that we know variants are being created in areas of uncontrolled community spread is again, like fixating on a leaking faucet on the Titanic.
Brazil, India and other counties with little to no immunized population and little will to control the spread is what you should be worrying about.