What you are looking for is very common in household
plumbing systems. In the US it's called a "tempering valve" and it mixes cold water in with the outlet of your hot water
heater to avoid just the situation you describe. A
plumbing supply place ought to be able to help you.
Here's a sample:
Tempering Valves - PlumberSurplus.com
Figuring out how to adapt them to your
marine plumbing system is your problem!
Bill