I am doing some repairs near the top of a pool shell and am sealing the
cracks in the shell behind coping tiles with a submersible, flexible
sealant. 3M makes both a polysulfide marine sealant and polyurethane
marine sealant, both of which would appear to be suitable in terms of
mechanical properties.

My question is whether anyone knows, or has had experience with, which
product might offer best resistance to any effects from constant
exposure to chlorinated water in a swimming pool. If anyone can answer
this (or has any other comments), I'd appreciate it.

Thanks.

tacker writes:

My question is whether anyone knows, or has had experience with, which
product might offer best resistance to any effects from constant
exposure to chlorinated water in a swimming pool. If anyone can answer
this (or has any other comments), I'd appreciate it.

Definitely 3M 5200 polyurethane for pool immersion applications.

http://www.truetex.com/pool.htm