The alternating hot/cold water problem is very common on many boats. The usual cause is the constantly changing water pressure when you have a on/off pressure switch on the supply pump. The accumulator tank helps, but really all it does is slow down the rate of change by allowing the pump to cycle more slowly. You'll know right away off this is the issue, because the temperature will change "in time" with the pump cycling.
A couple of things you can do that might help: Most of the water heaters on Amels have a "tempering valve" on the outlet to bleed cold water into the hot to control the temperature. If that is acting up, it is possible the temperature of your hot water is not constant, which would cause the problem.
If you have a tempering valve, adjust the temperature on it as low as you can. Typically people set these WAY higher than they need to be. If you set it just a bit hotter than you want for a shower (maybe 40-45C/105-110F) you'll find the shower is much better behaved.
Don't use the shower with the cold water "full on" and a trickle of hot to adjust the temperature. Set the flow rate just a little lower than "full on" and the temperature will change less as the pressure cycles.
The only complete fix I know of is to switch to a pump with a modern variable speed drive that keeps a constant water pressure.
Moraine Cay, Abacos, Bahamas