Engine temp gauges are sorta standard, between types - inoard motors to outboard motors. There is a gauge for each type as these 2 motor types run at different operating temps, with outboard generally running cooler.
The key difference is the sender that goes on the block, as these can be specified to your motor. Teleflex is the leader in this field. I have had great luck looking @ their stuff on the
www.go2marine.com website, search for the temp senders.
I'd say you do needs the sensors, as your warning horn/alarm system is only a wire that 'opens' at a certain temp, it does not read the temps at all. You will need a gauge and sender per motor.
You know ... if your OB picks up a plastic bag over the water inlets ... the powerhead will FRY long before the temp alarm goes off or you even notice the temp increase per the gauge. I went with a water pressure gauge too, in addition to the temp gauge. Thing is ... you need to watch this gauge like a hawk too.
Make it a routine to monitor your water pressure gauge ... I try to scan this gauge every time I do a quick look-around the boat while running and I monitor this gauge MORE than anything else.
They do make lower unit temp alarm sensors, for lack of water flow, but they are $600+ per motor.