I Am On Oxygen And Going To The Beach. Can A Portable Tank Be In The Sun Or Exposed To Pool Water?

It shouldn't cause any problem in sunlight if you are using it. Normally when a gas is heated it expand so never keep your tank full, there should be some margin for oxygen to expand and don't take them near fire and try to keep your tanks in a shadow

If filled properly your oxygen tank is only filled to 80%. That leaves 20% for expansion of the gas. Sunlight will not hurt the tank nor will pool water. make sure the tank is in the upright position always. The tank could relief if it was overfilled or left in extreme heat ( above 120 degrees ) for a long time. Keep it in the shade or place a towel over it.

I would advise you to keep it in the shade and be careful when using it on the beach if you are around smokers. Kabooooom :)