semisubmersible

Short definition from Wiktionary

Noun

1. 1
   A specialised marine vessel with good stability and seakeeping characteristics, often used in offshore roles such as oil drilling.
2. 2
   A tourist vessel that rides low in the water, with the majority of the vehicle submerged, with underwater windows.
3. 3
   A low-freeboard vessel, capable of being incidentally submerged, with the majority of the hull located underwater.

Adjective

1. 1
   Alternative form of semi-submersible.