What's a waterproof material to replace a wood pole with?

Any kind of metal tubing should work, but would likely be heavier (or much more expensive, if you want to match the weight and increase the lifetime). I've seen fence posts from recycled plastics used on farms around here, might be worth a try. Otherwise, you might look into more resistant woods - you didn't write which wood you were using, but my guess is that chestnut, oak, robinia (no idea if that's correct, Google translate), or, if you want to go to extremes, teak or bangkirai (or other tropical hardwoods - vheck your local supply for outdoor decks) should last much longer.

Recycled plastic gives me an idea - that's about the size of a surfboard mast. Glass/carbon fiber, lightweight (around 3 kg for 4.6 meters), almost impossible to break, won't rot in a thousand years, will cost you slightly north of 100 bucks each new - if you, by any chance, live/work near a windsurfing area, you should be able to get them used cheaper.

PVC Pipe seal the ends fill it with water or sand till its the weight you want it to be

You could try a 3" PVC pipe and fill it with concrete and maybe even a re-bar. Should be strong enough and will not rot or rust.

Use stainless steel poles. They will last forever.

You can coat the wood with flex seal. It will work.

try using a galvanized pole. no need to paint and will not rot.