i have a mate in far north queensland who grows in the mangroves,he puts grow bags up in the growth, with a mix of god knows what,and he has to go out in a small runabout boat,to check on them,.
personally there are alot of nasty critters live in places like that ,crocs,snakes and all kinds of creepie crawlies so fuck that,and he uses the water if he needs from the water there,adds no more to them than what he put in the first place,and he reckons he gets some nutes from all of the fish shit etc in there,it is a long way from where i live,and it would be full of mozzies,but not a single rip off,or bust and he has done this for many years.
it must work as he just bought a brand new harley??.
but i am not into saltwater crocs that would spin me out abit i think,plus as i dont live near that kind of shit cant really say,i know he takes a shotgun in case of crocs?i think snakes is enough for me,and there is plenty of those fuckers where i do my outdoor growing,dont like them myself but it is what you have here, so i dont bother them if i see them ,and vice versa.
i would have thought humidity,and bugs would be an issue,but he seems to do o.k,not my idea of fun things to do before i pop off though

p.s i think your tree idea is safer