I voted to keep it open until midnight.
I'd be happy to use a hottub in the garden pool area if it had one, but I don't think this is a very feasable option for you.
Another option is to keep Castaway Cove open until midnight only on certain nights of the week.....like during the busier nights of the week when you have more guests, as well as seasonally when you have more guests. During value season, possibly keep it open until midnight on Friday-Sunday, and close it at 10:00pm Monday-Thursday.
During holiday weeks, and high season (spring break, summer), keep it open until midnight all week.
Disneyland adjusts it's operating hours seasonally, as well as daily during the week, so why shouldn't you as well?