Mahogany is a good choice, after all there are still many many old Chris Craft run-abouts in use that were made with it.

We are in the process of cutting up a pre-WWII fishing boat that was planked with Mahogany and the wood is still as good as the day it was fitted. People are lining up to get their hands on the planks even though they have fastening holes every 6". Make sure you get true Mahogany, either Honduran or African.

A lot of places have Phillipine Mahogany, which is actually Luan, and is generally junk and is the reason it's approx half the price per BdFt... I use it for interior stuff but a lot that I've had splits extremely easily, so beware.
BC

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As an addendum to the previous post...I was unable to locate strength or weight properties for mahogany. I don't think it is very strong, light or weather proof. Teak is too heavy, it's good in the cabin and on the toerail with the overeaters.

While for appearance purposes you might select mahogany or ash, for unsurpassed strength to weight one would use clear vertical grain douglas fir. It is also reasonably rot resistant when painted. It probably shouldn't be varnished.

Ash is some 40% heavier for the same strength. Only White Ash should be considered.

Southern Pine may be more common in your area, it is a close second to DF.

Clear VG DF is commonly available as 5/4" stair treads (full 1" thick).

Many people suggested that I replace my recently broken spreaders with aluminum. Firstly, they would not have the original look. Second, they are complicated and expensive to retrofit. Third, aluminum sections are not much more efficient than wood. And, most interestingly, it was the *aluminum* tip that precipitated the spreader failure, the wood was sound.

If you varnish them, consider painting the topside a light color for weather proofing.
da