First choice after teak would be Ipe (tabebuia) as it is weather
and insect resistant. Second choice would be unvarnished and unstained white oak. White oak (quercus alba) has a long history
of being used in shipbuilding (USS Constitution for instance) and is very durable. It is weakened by reaction with iron oxide so use, bronze, brass, copper or stainless fasteners. Stay away from red oak (quercus rubra), as it is unsuitable for use in wet conditions.
As mentioned, Iroka wood from Africa
is a good substitute for teak, as is Japanese cypress (chamaecyparis obtusa).