Engineering

I'm late to the party, but I'm not sure why nobody mentioned the keyword "stuffing box". That's what this exact part is called in maritime operations, and as the name suggests it was traditionally a box packed with something to slow down the movement of water. You still have to bail on the other end, though, because it's not perfect.

However, since it's just a linear actuator it'd be simpler to go with an accordion-type covering. At shallow depths you could use a normal plastic one, if it needs to go deep metal ones exist. That's much simpler to deal with than an actual sliding bearing.

If the travel distance of the rod is fairly limited, you can use a corrugated boot.

Otherwise, maybe a gland seal with positive air pressure on the dry side?

If you have an air line to the surface, then running the enclosure at positive pressure to negate the force of water pressure at depth might be a starting point.

I would also reconsider whether a linear actuator is the right approach. A hydraulic piston external to the housing where your motor resides reduces the complexity of any waterproofing, and might be a better approach if you're dealing with significant depths/pressures.