The colon is already colonized (no pun intended lol), with billions of bacteria that compete for space and resources. I would not worry too much (if at all) about how contaminated the water might be, unless you had a perforation - in which case you would be in deep doo-doo anyhow. However, I would be extremely concerned about the osmolality of sea-water, as some one else pointed out, water follows salt, so if the salt-water enema had a higher concentration of salt than the blood (quite possible), you would end up becoming even more dehydrated.