Compleatly dead but batery charged

I've got an engine issues that that has completely stumped me so I thought I would look to the experts. I took my 1970 Ski Nautique out tonight and the engine cranked right up first try on the ramp. I ran to the other side of the lake and turned it off and it wouldn’t start. the first time I tried it, it was close to turning over and if I had given it some throttle I think it would have gone but the second time I went to try and start it, it was completely dead. Everything was dead, all the electrical stuff, the horn, blower, all the lights. It was as if the battery was disconnected. I checked the battery connections and they were all tight and the battery had a full charge on it before I left so it was not a dead battery. The electrical power came back on later but the engine still would not even turn over. Eventually it turned over and started right up when it did. When I got back to the ramp I turned it off and then tried to turn it back on just to see if it did it again and it was completely dead again but then on the 5th or 6th try it started right up. Do you have any idea what might be causing it to go completely dead? because I’m at a loss. I’ve replaced the alternator, and the starter recently and I’m about to replace the Voltage regulator. I really appreciate the help!

Clean and check your battery posts and cable terminals. Check voltage at the battery and what you get to the starter. Is the start relay energizing? If not, check the voltage going to it via the ignition switch and the NSS.

Finally had some time to work on the boat yesterday, turns out I had a bad voltage regulator and the batery wasn't getting a charge.   Thanks for all the responses, I really appreciate it!

Did this fix the problem?