I do this due to a small leak
in one of the seals that allows about a half cup of water in the same
time period

If you are running your foot with that much water in it I doubt you really care
what type of lube you are using. You will quickly overcome any extra additive
(if it is really there).
Bought in quantity the marine stuff is about the same price as a name brand
80w90 anyway so I doubt there is really that much difference.