Hi, I've written about the specific 50% claim before in this post (by me, /u/superplaner, I see you automoderator). The TL;DR-version is that a 50% mortality rate was expected for "major voyages" with no specification for what a major voyage is. I think we can however assume that most deployments did not contain "major voyages". It is probably realistic for long term deployments far from home where access to fresh food might have been extremely limited but not in areas where friendly ports to resupply in existed.

The second part of this question is, they were appointed. A man did not generally, or ever that I'm aware of, become an admiral by starting out as an able seaman and rising through the ranks to admiral. Admirals were almost without fail noblemen with a naval career of varying lengths behind them. Without going into specifics, a common career path would have been to start off as a junior offier on a deployment or two in their youth, then spend some time as a staff officer at a nations admiralty or equivalent, then perhaps a major command as a senior officer followed by promotion to admiral.

It should also be noted in this context that conditions for the officers were generally much better than they were for enlisted men. The better food and quarters in particular means they would have suffered a great deal less from things like scurvy and typhus or other diseases.