Your headset is very likely use modern cartridge sealed bearings which are designed to operate for thousands of hours at hundreds to thousands of RPMs. A headset turns 1/2 a turn (at most) a few times an hour. That’s a very low duty cycle and doesn’t require any kind of maintenance for years in all probability. Even then, the bearings are meant to be replaced, not serviced in most cases.

In other words, leave them alone and don’t worry about it…even if you are riding in water 100% of the time.