You missed my point , with engine failure they can cause
water to enter your heads , my defective valve springs
caused this , but can anyone think of a way to modify pipes to prevent this ?

Headers and tails almost new tested fine !

You could always customize some flappers if you are that worried about it.

What kind are those and where did you get them??

I had a set on my last boat had no issues, really helped. However I was a little concerned with the extra force (vibration) that was being applied to the exhaust tips which could be transmitted up through to the headers. I weighed mine and did some math on the theoretical force being applied through leverage (lost the napkin) and it turned out be more than I expected. Concerned with long term effect, if any.