Use the marine versions
I've read the specs for timbertech--One of its components is "wood flour", i.e., sawdust, which makes it suspect for our application because we don't know a. how long the manufacturer intended it to be in a state of constant immersion, and b. how it will react to salt water. It's best to use products made for the marine industry, such as Flexiteek, Marinedek, and Tek Dek. Flexiteek and Tek Dek are plastics which were developed for commercial boat builders in Europe needing a long-lasting non-skid surface on passenger-carrying boats that also looked like planking. Marinedek is used in a similar way but is made of cork composite (doesn't look as bad as that sounds).I've ordered some Flexiteek for my deck repairs because their sellers put the piece together from your dimensions rather than making you put the planks together yourself, as Tek Dek seems to do. It depends on how handy you are...There's a company out there making synthetic teak-and-holly flooring sheets and imitation teak lumber, but I couldn't find their name. If you can go without woodgrain then Star Board is always a good material for non-structural bulkheads, hatch covers, etc.