The traditional hat-to-boat from a rectangle forms a regular part of my teaching classes. It’s fairly simple, involves some nice “in the air” open & squash folds and makes a superb boat with sail. I also regularly take newspaper with me to deliver the classic “captains shirt” story, which never fails to amuse young and old. There’s some more useful info about the model here.
One method I often employ to reinforce learning of a design is to have students fold it again from paper half the size. They then repeat this process and a friendly “how small can you go” competition often ensues. The boat is perfect for this and you can also introduce the concept of A4 / A5 / A6 etc. if you wish.
The boats also tuck neatly into each other, providing you fold such that a decent sail is produced. I’m sure you know, but the ratio of sail to hull is determined at an earlier stage, when you fold up lower corners.
Folding to the top produces a sail level with the top of the hull. Folding less than this distance produces a sail. Eventually, the hull is tiny and the sail huge. Encourage the students to investigate the possibilities.
A few years ago I discovered a development of the boat, one which I haven’t seen before, although the chances are it’s already “out there”. You perform another “open & squash” move, swinging each end of the hull in opposite directions and you have a fish. Neil Calkin points out in his comments that you can take it at least one stage further, producing a kind of gondola, although this may well be beyond the capabilities of a novice folder. Further iterations will depend on the thickness of your paper!
These ideas further extend the possibilities of this sequence, (providing you don’t tear to form the t-shirt!) allowing you to make “sea” montages by using a large sheet of blue paper, drawing a line to represent the horizon, and adding boats (and birds) above the line, fish below. A sharks fin is easily formed by folding a square in half, try setting this as a creative challenge.