I agree that this is worth a complaint. It would not be difficult for the hotel to note that the pool is reserved at said time for a special event before your arrival. They did not do that. The pool is marketed as a public venue open to all guests. If the hotel wants to close it to sell it to other guests, that’s within their right to do, however it’s going to cost them. Imagine how much money they made by selling that space and inconveniencing you in the process. They can as absolutely afford to pay you compensation for this lack of hospitality.

I actually like the idea of crashing the event though. Could be interesting! Lol