SCUBA is a sport that requires a variety of gear. A boat is one such item that might be purchased. Here are some things to consider when comparing types of diver-friendly watercraft.
Rigid-Hulled Inflatable Boat
Often referred to as a zodiac, these boats are most often used for relatively short jaunts for small to medium-sized groups. The boat is usually shape like a narrow horseshoe, with stiffly inflated sides and front, a rigid bottom, and a motor at the back. It is usually transported on a trailer, and not deflated in between each use. They can be launched from many types of locations, depending on weather, and the smaller versions can even be transported by a group over rocks or sand, if needed. Divers typically exit and re-enter the boat by going over the side.
These boats provide no shelter from the elements, and are therefore mostly used in temperate or tropical climates.
Open Rigid Hulled Boat
These boats are similar in size and function to rigid-hulled inflatable boats. They are typically heavier, for the same size, and are less stable for exit/re-entry, as they do not have the ballast of the inflatable sides to provide stability. These boats are often more economical, more durable, and are frequently made of aluminum.
Fully inflatable boats are good for very small groups for short distances that are easily gotten to by muscle power. Typically, they are the least durable of all boat options, though their portability is the most of any boat as they can be fully deflated.
Possibly the most frequently seen, these boats are usually kept on a trailer when not in use. They require a motor vehicle to transport over land, and a suitable place to launch from. These boats are typically 20 to 40 feet (6 to 12 meters) long, and can be used for medium-haul trips. Some have platforms at the bow or stern to provide passengers with easier exit/re-entry, and almost all models have ladders to lend aid. Some models also provide a small cabin area, mostly useable for storage or sleeping. They are usually made from a rigid material like fiberglass, and are very durable. Most use outboard motors.
Day boats are typically much larger than pleasure boats, and require a slip in a marina for storage. They are typically made from fiberglass, metal, or wood, and are usually 40 to 90 feet (12 to 27 meters) long. They have inboard motors, and are steered by a wheel in a sheltered bridge. These features mean that they are more limited in the places they can go, compared to the other boats mentioned above. However, this limitation is made up for by providing some shelter and more comfort facilities than smaller boats: a larger deck, and a below-deck area that can contain seating, a galley, and a restroom.
Trips on day boats can last as long as the whole day (or night), but passengers do not typically sleep on-board.
Other Types of Boats
Basically, any kind of watercraft can be used to facilitate a dive trip; fishing boats, schooners, yachts, sail boats, jet skis, etc. Important details to factor in, besides functionality, are: cost, transport, storage, and safety.