This is what we found on Wikipedia regarding the Thames Cruise: Westminster to Greenwich with optional return:

A boat is a watercraft of a large range of types and sizes, but generally smaller than a ship, which is distinguished by its larger size or capacity, its shape, or its ability to carry boats.

Small boats are typically used on inland waterways such as rivers and lakes, or in protected coastal areas. However, some boats (such as whaleboats) were intended for offshore use. In modern naval terms, a boat is a vessel small enough to be carried aboard a ship.

Submarines are referred to as boats rather than ships regardless of their size.

Boats vary in proportions and construction methods according to their intended purpose, available materials, or local traditions. Canoes have been used since prehistoric times and remain in use throughout the world for transportation, fishing, and sport. Fishing boats vary widely in style, partly to match local conditions. Pleasure craft used in recreational boating include ski boats, pontoon boats, and sailboats. Houseboats may be used for vacationing or long-term residence. Lighters are used to move cargo to and from large ships unable to get close to shore. Lifeboats have rescue and safety functions.

Boats can be propelled by manpower (e.g., rowboats and paddle boats), wind (e.g., sailboats), and inboard/outboard motors (including gasoline, diesel, and electric).