temporary swimming line in community pool

Most of the time I swim in a community pool which has sufficient length but lacks a line on the bottom to keep me swimming straight. Fortunately I am usually the only person there and have some freedom for what I can do, so I have tried laying rocks but they are usually difficult to see unless I lay a large amount which is tedious to do each time. I researched pool lights and most of them float and/or have crazy patterns. I just want a simple light which I can affix to the bottom of the pool quickly, and then remove when I am done. I prefer battery power as I am not too keen on putting a live power cord inside the pool. Does anyone know of such a light, or any other solutions? Thanks.