Heres the skinny,

I want to bring a trombone into the ocean. BRIEFLY. I know that saltwater is probably one of the worst things for a brass instrument. Can anybody help me and tell me how long I have before I will notice a significant difference in the horn if bring it in the bay for 15-20 minutes at the MOST.

My plan is to rinse the trombone off in the shower. Give it a good back afterward. What are your thoughts?

No problem in my opinion.

The only caveat I would advise is that if you have a valve on your trombone, don't use it in the ocean or bay. The salt or other suspended debris in the water could damage the valve in the short time it is in the water (the same could be true for the slide' so don't use the slide either.

If you are going to do it though, don't use the valve. Clean the slide before you go to the water with Dawn detergent. Get all the lubricant, slide grease and oil off it. This will make your slide tolerance larger while you are in the bay. If you have a Bach or Conn you might be okay. If you have a Yamaha, use a different trombone (perhaps a friend you don't like anymore).

When you are done, rinse the trombone well.
then submerge it in the bath tub and jiggle it gently very now and then.
Rinse again.
Then go through your normal cleaning routine.

Why do you want it in the ocean again