There should be an feature which allows you to add players as friends so you can keep track of them, whenever if they are playing on this server, or if they are offline or online, or where they are in the server. You should also have a feature to block other player's requests and unfriend people if there has to be the occasion to. What do you think of the idea? P.S. Whoever that player I was playing with in SMP 4, are you still there? Did I drown you by mistake?
I don't know if you know this yet but /p playername Will tell you quite a bit of information such as what server they were last on and how long ago. I think it may also say if they are in town, wild, etc. if they are currently online
Live map does the job your taking about and I think can't remember the Tab button tells you who is online on the server. Also I have drown countless times at a wild base building under water structures