Many fish have a protective coating (slime). If you touch them, and disturb the coating, you can cause them to become susceptible to diseases, and possibly become ill and die. Touching other things (fire coral and things mentioned in other posts) will make you wish you hadn't. I've had DM's who have picked up sea cucumbers, arrow crabs, and other creatures, but the best rule of thumb is to not touch anything. Grabbing hold of (or kicking) coral can wipe out decades of growth. As it's been written, "take nothing, and leave only bubbles." Remember, you are a guest in their world and need to "mind your manners."