I'm not an appraiser. I don't know your code enforcement regulations in your area, but in our area of S Florida there has to be a fence around the pool (not just the yard) and at least 2 (maybe 3) "barriers" if entering the pool area from the house. "Barrier" is also considered an alarm and locking door. Fence has to have a locked gate. This is for all pools, empty or not.
In Fannie Mae and Freddie Mac foreclosures they board over the pool - water and all - for safety reasons. It's ugly too. And that orange construction barrier type fence is around the pool area in addition to whatever other fencing was in place at the time of foreclosure. The last one I sold in this condition the buyer was a cash buyer and there were all kinds of surprises after they removed that plywood board over the pool. No board or covering was removed until after closing. Fannie wouldn't allow it.
Naturally for FHA it is, IIRC, the pool has to be blue, clean and clear before the lender will fund.
So I have no idea how your appraisal if affected by a empty pool for Fannie Mae. Only how they deliver it when it's a foreclosure.