On some occasions you don´t have anything to desinfect wounds, for example during a (short) walk along the beach. If you cut yourself or have a an small wound that needs cleaning, would it be wise to clean it in the sea or better to leave it the way it is?
Seawater is not itself sterile, in fact, it has all kinds of organisms. Salt in a wound is likely to hurt, and saltwater won't be a particularly effective antiseptic.
With wound cleaning, it's always a question of trade-offs. It might be better to use less-than-sterile water to clean an extremely dirty wound if infection is otherwise inevitable. However, whenever possible you should only use clean water in a wound, so you don't introduce more possibilities for infection. If you do have to resort to salt water to get mud or large debris out of a wound, definitely rinse with any available clean water afterwards.
Your best bet is to use some purified water (such as from your water bottle) to rinse the wound, then clean it more thoroughly when you get to appropriate infrastructure to do so.
No, seawater cannot be considered antiseptic. At minimum, Staphylococcus, which is the cause of many serious infections and deaths every year, and one agent of necrotizing fasciitis, is "able to survive: ... High levels of salt" and may even be spread by seawater.
I recall reading (but am presently unable to cite) that parasites such as Cryptosporidium cysts can also survive in seawater.