Brian Whatcott wrote:

Place three in series in the housing, and fix the housing on the motor
in some waterproof way. This will put you in with a chance.

That is not a very promising approach.

If keeping the motor dry is important, filling the housing with a light
oil under slight positive pressure via a compensating bellows is the way
to go. A standard lip seal will keep junk out and the oil in. The
compensating bellows will allow operation to any depth with no fear of
flooding the housing.

Rick