It may have taken the castaway/ castaways a while to work their way up to the forecastle/ crew area, exploring for resources along the way. If you look at the debris from the collision with the bridge you will see all sorts of stuff. Pulleys, cables, steel ladders, winches. A big gash was in the hull to pass in and out thru. From the NZ pictures, the forecastle area still looks intact though.
Maybe they started at low tide and got trapped for 12 hours during high tide. They could have slept for a while. Though it may have been difficult to do with loose steel and debris getting pounded by waves. Then they likely had extra time to paint while waiting for low tide to return to the beach. They had flashlights to work at night, and probably bought them to explore the ship even in daylight. The paint and brushes could have simply been in a locker near the bulwark ready to use.
If they did decide to make camp on the shipwreck, It may be a 12 hour cycle you had to work around. They would have lots of extra time during high tide.
One good thing about the ship as a camp, is the fish are easy to catch. Eric Bevington said “I boarded the wreck and found the hold to be teeming with mullet; they were so thick that more fish were visible than bottom. The natives easily speared them” In fact the food on the island seems really easy to catch anywhere, it crawls right up to you. The problem is the cooking. Maybe during the day they collected fire wood, then return to the ship before nightfall and high tide. There your fresh food awaits.
You could spend the rest of the time sleeping or looking for help from a very high point. Your stash of fire wood ready to light. Maybe a fire set in a bucket ready to run up the mast at night.
They could also rig a gutter or simple rain diverter at the bottom of the forecastle deck where it changes height, and could catch a lot of fresh rain water running down the forecastle deck. This could take very little effort. Place a board or 2 at the bottom of the deck and put a bucket next to where the rain is diverted. Maybe the ship already had holes for water to run off the deck at the bulwark. Just hang a bucket under one.
Water, food, shelter, high point for signaling and viewing. Maybe the shipwreck is not a bad site after all. I just think it would be hard to get in and out of on a regular basis.
Did they have a raft for getting around the lagoon and reef?