After driving through water the 300tdi motor on my Defender 110 makes a squealing noise - I understand that this comes from the tensioner bearing and after about a week of spraying it with WD40 it goes away.

Are there any modifications that I can do to stop this from happening in the first place? Like an aftermarked bearing that does not allow water to ingress or something like that?

Cheers

Don't drive through water?

Have you heard of the penny trick (or washer trick)?

Basically the tensioner wears slightly eventually letting the bearing sit slightly at an angle to the belt, thus the belt squeaks, it happens more so after water spray because it acts like a lubricant and helpts the belt slide over the sruface of the pulley.

Undo the tenioner to engine bolt, and insert a penny or washer in behind it on the drivers side and do the bolt back up.

That should stop the sqeaking.