I have a 20 year old boat and at start of this season I had all the Heat exchangers off each engine. They were taken away for cleaning where it was discovered that 1 oil cooler was about to fail - Sea water and oil not good.
Yes I had it changed and had to swallow the bill for the part. The total labour time was 4 full days . I know this as I was helping my engineer take the old parts off and put them back on again. When we put every thing back together we replaced "EVERY" jubilee clip on the cooling circuit on both engines. - Was this worth while ....... For my peace of mind YES . Some had never been taken off and on the surface looked fine with their green Volvo paint on. But when checking the torque 2 of them failed - just fell apart. so the other engine had these replaced regardless,
I can now use the boat knowing that what I have done should not fail.
Do I change impellers each year. yes as for me the boat is dry stacked and they can dry out. Plus the engine bay gets very hot. What I do not want to happen is an impeller to fail and bits block up the cooling system.
I do all this because I Can NOT afford for a major engine failure. It would write off the boat.
As others have said. Each to their own and I for 1 would not tell others how to look after their boats.
When I sold my Fletcher after 6 years of ownership from brand new I had a stack of invoices for EVERY job ever carried out on the boat , These were detailed down to the last nut bolt and grease used, I received a lovely letter from the new owner saying how much he was enjoying the boat and could see that it had been very well looked after.
He went boating with confidence knowing that nothing had been left to chance and filled him with confidence that he was going to use the boat and enjoy. NOT having to worry if it was going to break down due to neglect.
YES things do break down when not expected but I try and reduce the chances by following the guide lines laid down for service ETC.