Flying is all about risk management and in an SEP there are obviously risks flying over water.
You can minimise those risks by not flying over water on windy days where those pretty white bits 2000 feet below are 20 foot waves close to.
Fly as high as you can! Note the position of boats and ships and wind direction
The chances of a complete engine failure are small and more likely a loss of power so keep an eye on the engine gauges , fuel quantity and if you have carb heat check regularly.
Don't set off late in the day where should you go down there is a short time till darkness and search and rescue will have problems
Talk to someone and preferably be on radar
But it is risk management and some are prepared to take more risk than others
I don't like SEP at night and must admit admire the guts of those who do long ferry flights in SEP planes over the North Atlantic
Unless you stay within the UK you have to fly a water crossing and I have done in the past many times but its amazing how a normally smooth engine always sounds rough over water especially the mid part
There are irrational fears where we can talk you out of it and get you to look at things in a different way and there are rational fears. This is a rational one so all anyone can do is minimise the risk and take the chance how big a chance or how long a crossing is up to you
Or reduce it further by flying a Cirrus
Pace