The following is a non-exhaustive list of vessel types defined in various IMO instruments:
A passenger ship is a ship that carries more than twelve passengers. (SOLAS I/2)
A fishing vessel is a vessel used for catching fish, whales, seals, walrus, or other living resources of the sea. (SOLAS I/2)
A fishing vessel means any vessel used commercially for catching fish, whales, seals, walrus, or other living resources of the sea. (SFV 1993 article 2)
A nuclear ship is a ship provided with a nuclear power plant. (SOLAS I/2)
A bulk carrier is a ship generally constructed with a single deck, top-side tanks, and hopper-side tanks in cargo spaces. And is intended primarily to carry dry cargo in bulk and includes such types as ore carriers and combination carriers. (SOLAS IX/1.6)
A bulk carrier means a ship that is intended primarily to carry dry cargo in bulk, including such types as ore carriers and combination carriers. (SOLAS XII/1.1)
Oil tanker means a ship constructed or adapted primarily to carry oil in bulk in its cargo spaces and includes combination carriers, any "NLS tanker" as defined in Annex II of the present Convention, and any gas carrier as defined in regulation 3.20 of chapter II-1 of SOLAS 74 (as amended) when carrying a cargo or part cargo of oil in bulk. (MARPOL Annex I reg. 1.5)
General cargo ship: A ship with a multi-deck or single-deck hull designed primarily for the carriage of general cargo. (MEPC.1/Circ.681 Annex)
A high-speed craft is a craft capable of traveling at high speed. (SOLAS X/1.2, HSC Code 2000 para 1.4.30)
A mobile offshore drilling unit (MODU) means a vessel capable of engaging in drilling operations for the exploration for or exploitation of resources beneath the sea-bed such as liquid or gaseous hydrocarbons, sulfur, or salt. (SOLAS IX/1, MODU Code 2009 para 1.3.40)
A special purpose ship (SPS) is a mechanically self-propelled ship that, because of its function, carries on board more than 12 special personnel. (SPS Code para 1.3.12)