If you decide to do it, I would second stainless as preferable to aluminium. My boat came fitted with stainless (by the way).
The reason against Al is that even if tempered to T6, it still disturbs far more easily then stainless. Disturbing is a blacksmithing term for compression loading that causes the pipe to mushroom and get shorter, but develop thicker walls in some spots or in other ways redistribute the metal. For a compression post, it does not need to get much shorter before it is doing virtually nothing at all.