From its structural properties a magnesium beam would be quite good, however magnesium is very sensitive to corrosion, especially in salty environment.
In theory beryllium has excellent properties, but it is difficult to handle, it may burn, when milled and it is higly toxic.
In aviation the most promissing metallic material is alumium-lithium alloy, because of its low density, and the hybrid alumium glass fibre (fibre metal laminate). However environment and load cases are different to what we see on beach cats.

At the end of the day you need only focus on alu, glass, carbon and wood as materials for small cats, without missing any significant benefit. You can improve the design much more by good engineering, rather by material choice (e.g. not a constant layer setup for the whole hull). You can have a huge benefit by swithching from polyester to epoxy. However exotic materials are good for marketing.