Will this be stable enough as far as warp is concerned?
The term 'warp' can cover a lot of issues. Your boards may readily cup, twist, bow, or crook due to the repeated exposure to water and uneven drying.
The amount and type of warping will depend largely on the arrangement of the grain as much as how the boards attach. In general a short stub tenon at the corners will do little to resist most of these categories although it will resist cupping at the intersection areas even though it may occur further toward the middle of the boards. Your best may be to select individual boards with very straight grain. You can reduce cupping by selecting boards with vertical grain orientation relative to the wider board dimension. You might also want to mount the this trim frame to block risers rather than directly contacting the pool liner directly to maintain some airflow around the frame allowing more uniform drying.