What converter(s) do you use for rs485?
Sorry, been away for cupla daze.
I'd be looking at the LTC1487, MAX1482, ADM483 etc.
Ccc56: As CD says 485 is only a phyiscal layer you still need to write a protocol and AFAIK there is no simple standards. From what I can see you have a very simple application with no control, just monitoring. Not having control makes a big difference, you can get away without massive security.
I'd just have the remote Ardiuno send a few bytes indicating the status of the bilge etc. Add a checksum and maybe a frame sequence counter so the receiving Arduino knows if there's been a stuff up in the transmission. Also have a timeout so you know if the link has failed.
The worst case then is that you get too many false alarms which either isn't a big problem or then you look into hardening up the protocol.
The receiving Arduino then just talks USB ot the PC.
If you want to get some idea you can have a look at one of my current toy projects (http://busnet.robgray.com), this is overkill for what you need I think although if it was a going concern I would suggest it would do nicely.
Rob