Is a weather resistant gfci required outside in a metal box with in use cover?
A WR GFI uses stainless components and screws. GFI's are notorious for rusting, especially at the mounting screws. The one that went bad was probably older, before WR's were even available.

Yes, you need a WR GFI, and an in-use cover. In fact, I'd venture to bet that much of your installation is not up to code. IF it is that close to the pool the receptacle and plug must be of the locking type (twist-lock), and the circuit protected by an upstream GFI. The outside portion of the wiring to a pool motor must also be in conduit with individual insulated conductors. Older codes would allow this installation located between 5-10 feet from the pool, but NO closer than 5'. Also, the cord on the motor should be no longer than 3' and must be #12 or larger. 

There are MANY codes involved with pool installations, I hope yours meets them.