wooden pond top material

[koiman]

I have a pond built above ground. Its currently topped off with 10" x 1" varnished mahogany. I have never been able to get the varnish to hold for any length of time, it always peels off.
Last time I did it I sanded right back to fresh wood and applied four coats of yacht varnish, it still peeled after just 12 months.
Q1 is there a better finish to use for longer life?
or
Q2 is there a more suitable wood (at a reasonable price given that I will need at least 40 feet of circa 10" x 1")?

[The Builder]

Koi ......... a modern microporous (breathable) woodstain would do it, but since you have the colour of the mahogany already, why not try just a clear preservative ........ Cuprinol or similar. Applying a "film" such as varnish often results in it lifting off eventually. A preservative just soaks right in.

A lot of trendy modern buildings are clad in Larch. That isn't treated, and just weathers into a natural patina.

[buildersmate]

.. or Cedar as an alternative treated in the same manner. Not sure on price though.

[jomifl]

all varnishes will eventually degrade, but some degrade quicker than others. They adhere best to dry wood. If the wood is varnished on top but the underneath is exposed to the pond water the varnish will stop moisture escaping and the wood will become too moist for the varnish to adhere. As previously suggested a microporous mahogany coloured paint is probably your best bet.