General aviation air taxi pilot reported a loss of control during landing on a designated off airport beach location. Pilots evacuated safely with no damage to the aircraft.

Narrative

Another pilot and I went to the beach near ZZZ [airport]; on a Part 91 beach training flight. We do this every year prior to beginning bear view season. I needed a beach refresher and the pilot with me needed their initial beach training. I was in the left seat so that I could do my beach refresher; and then my plan was to swap seats and train the other pilot. When we arrived at the beach; I initially did a low pass over the entire beach. I noted debris low down on the beach in a few places. It was hard to tell how smooth the sand was. The beach has a fairly straight NE/SW section before it curves into a more curvey E/W section. I chose the straighter NE/SW section for my initial landing. Before landing; I dragged that section of the beach. It felt firm but I little rough. I decided it was landable and chose to sand SW so that I could taxi past a log low down on the beach before turning up the beach so that I could turn around. I had a successful landing. As I was rolling up the beach to turn around; I encountered soft humps in the sand that the plane was able to taxi over; but it was not a good beach condition and I did not think it was a good spot to train the other pilot who had never done beach landings. I was able to get turned around and take off to the NE. I decided to check out the E/W section of the beach to see if I could find a smoother spot to train the other pilot on beach landings. I picked out a section and landed. This is where I think I made a mistake. I should have dragged that section of the beach before landing. A low pass is good; but dragging it would have given me a much better view of that section of beach and allowed me to abort if I had seen an issue. I did not drag it though; and while I was rolling out from my landing; a pit in the sand suddenly appeared ahead of me. The plane was able to roll through it; but the sand just past the pit grew progressively worse. It was soft and undulating. The combination of the slope of the beach and the soft undulating sand forced the plane down slope toward the water. I did not have enough speed to give me the rudder authority to turn the plane back up the beach and I was struggling to use the nose wheel steering in the soft; undulating sand. I was nervous to add power or slam on the brakes at that point because I felt that the nose wheel might dig into the sand. I ended up rolling out into the water. The other pilot was able to get on a boat back to ZZZ [airport]; and I; with a lot of help from nice people on boats; was able to get the plane out of the water onto the beach. There was no visible damage to the plane. Myself and my fellow pilot were unharmed.