Hi Simon and welcome.
Given the ethos of this forum, you should not be short of advice on this subject. Some points from me:
1. Try to get the fish back into the water as quickly as you can. It is possible to "process" the fish for a photo and release in 30-45 seconds.
2. Handling is extremely important. Always support the fish with both arms or even better lie it across your lap. Never hold the fish up by its tail and never touch the gills. Keep handling to minimum as it is very easy to remove large quantities of the protective slime from the skin and this leaves the fish open to infection.
3. As you mention, it is good to have a salt water hose on board to pump water through the gills. If that is not possible, use a bucket.
4. When releasing the fish, do so slowly and gently. If it does not swim away immediately, support it in the water moving it gently until it fully revives. Never throw the fish back into the water.
I am sure others will have more advice.