Can an above ground pool be repo'd?

Pretty much anything you pledge as security for a loan can be taken if you fail to pay as agreed. That's the point. Or, if a creditor gets approval from a court saying they can take some specific item (normally because you failed to pay a judgment or such), that is pretty much the same. You would normally be responsible for any costs the creditor had in getting or enforcing the right, including the costs of repossession itself, too.

In something like this, there may be some matters about what they must do with the water in the pool, etc. to be dealt with. If you used the pool as collateral for the loan, from a company that regularly handles these types of things, they probably already covered that in the loan agreement, with they have to use care, but are not responsible for it or the damages it may cause. Read it.