Last week I fell in standing water and completely immersed my 1911, nothing to worry about. When I was done for the day I took it apart completely, putting the parts in the toaster oven to dry. Then after drying it was lubed and reassembled.
Most ammo should be fine, especially factory ammo. Not only are the tight tolerances to small for water to get to the powder and primer, when the bullet is loaded air is compressed in the case making it nearly impossible for the ammo to get wet. I have accidentally washed 38 ammo in the wash machine, and pulled the bullets to check for wet powder. They were always dry and when reassembled went bang.
If though you are going to be in salt water, and the gun gets wet, keep a bucket of fresh water to submerge it in, after shaking off the salt water. Leave it in the fresh water long enough to dilute the crevices with fresh water.
As said use a kydex, nylon, or a hot wax treated leather holster for water use. Leather treated with beeswax or even paraffin wax will be as hard as kydex, and waterproof. I treat my cotton web belts with paraffin to make them stiff for use with belt slide holsters. A leather belt treated with wax will be as stiff as a kydex belt. I also make my cotton shoes waterproof by the same method. Keep in mind that leather and especially cotton treated with wax is very flammable if exposed to flames.