Dove would have been my first choice... As in, "The pelican dove into the water.". "The pelican had dived into the water, but resurfaced fishless soon after." Is how I would use dived.
As for car planes or boats, it doesn't really matter... a firewall is one purpose of a bulkhead, pressure is another as is water.
A bulkhead is by definition a structural panel , usually vertical in orientation, and the preceding words ie fire resistant/proof; pressure resistant/tight; water tight etc indicate how the bulkhead is designed and engineered to perform. Ergo a firewall is a bulkhead that's designed to contain or slow a fire. Depending on how it is constructed and from what, it may serve more that one purpose.
On a ship, a watertight bulkhead may also be designated as a fire wall as it will have been designed as such, albeit a secondary requirement to being watertight... If a ship sinks because it flooded then having firewalls is a moot point. But equally if a bulkhead cannot contain a fire then it is unlikely to be watertight. And if a compartment floods the watertight bulkheads serve as pressure bulkheads as they have to resist the weight of the water behind them.