Underwater video is over-rated. Well done over the water video can show damn near everything that needs to be seen. Underwater video is for micro managers.

The end of the lane is not the best place to do this, as there will be no direct side shots, and the majority of the video will be of a very far away swimmer. Try for an end lane, camera in the middle of the length with the person on deck. Stay still. Let the swimmer come to towards you, pass you, and swim away. Hold steady. That's it.