I have an old boat.  Mercruiser I/O with 454...  edlebrock carb'd

 I want bilge detection.... flood, fire, gasoline vapor?   or just gasoline vapor?

I'd like all of the above...anyone have favorites?

thanks!

Xantrex seems to be the most popular. I rarely see one fail as far as the vapor detectors. For fire supression, the Firebouy with a manual secondary cable operated trigger seem most reliable. These use the little glass stopper on the bottle/nozzle itself and a manual pull cable at the helm with lever on the bottle to break the glass stopper if it doesn't shatter from the heat first.
High water alarms can be pretty simple. Hooking up a warning light or buzzer to the auto-float wire that goes to the pump is easy.  Some pumps are wired with a lighted switch on the dash which will light up when the pump is running even if the switch is in the off position.

I don't know of any fire detectors, but there probably are some.  Fire suppression is spendy, but it's good peace of mind and gets you an insurance break in many cases.  You can find used systems on ebay and other sources.  Flood detection is simple, there are systems or you can make your own.  My set-ups have always been an automatic bilge pump with a dash light and a secondary automatic a little higher with an audible alarm.  I wire one pump to each battery bank to have two completely separate systems, fewer possibilities of loosing all pumps.  To add a light or alarm just tap into the switch output/pump input power wire.  Make sure the connections are well sealed.  Always fuse the power line to the pump manufacturer specs.  The fuse is always illogical for the wiring but follow the recommendation.  The reason it's such a low amp fuse is because it's protecting against a short within the pump where the wires are tiny, an internal pump short can melt the pump and start a fire.  You could also wire an alarm to an independent float switch without a pump if preferred.  Test switches, alarms, and pumps regularly to assure they're functional.

thanks!

I've picked up a gas fume alarm and high water alarm..... I think I'll get a CO detector and I will feel secure with the family onboard for overnighters,  etc...