found this chart from SKF (bearing producer) on friction loss from shaft seals, it might not be very accurate but the loss seams to be quite minimal.
just for fun a manual calculation, the friction and contact pressure, is a bit of calculated guesswork, based off some tables I found online. Found about 50w of loss. But I think it is a bit less.
in my opinion, power loss from a shaft seal is acceptable. Having pressurized air to prevent leakage might work but way to complicated. Just fill the compartment completely with grease and no water will ever enter. it also reduces noise, keep it simple.