It is possible to use brass/bronze sleeves under water (depends on proper alloy selection) but probably not the front-up choice for a pool. Plastic sleeves, even though they probably would not last as long as metal, are more inert and thus a better choice.
As for pole material choice - will this be a salt water pool? If so, then you should not use bare metal poles. If the pole is made out of aluminum, you'll want to ensure it either has an anodized coating on it (nano-scale, porous aluminum oxide coating that is electrochemically formed and can be colored) OR that the aluminum has a high quality powder coating (thermally sprayed oxide coating). Bare aluminum in salt water will corrode over time at the waterline around the pole. Anodizing or powder coating creates a barrier to corrosion. Cheap painted pole coatings will not last more than a season or two.