I can't imagine a sailor that is not interested in the biology of the Chesapeake, though lamentably, a few exist.
Field Guides. Bring a few. Though I'm not a bird water and have no obsession with memorizing the name of every bird, fish, or reptile, a working knowledge is fun. Guests always have questions and often get a kick out of finding their own answers. Sometimes I feign uncertainty.
In truth, for all of human interest in macro fauna, micro organisms are
responsible for most of the important chemistry and much of the world we
live in. Thing like oxygen.
Aquariums. A definite plus when kids are on-boards, right up through college if they are science-minded.
2.5 gallon hexagonal aquarium on-board, studying what ever we found for a few days be fore returning it to its home. The under gravel filter works well in salt and fresh water. It is required to trim the lift tube about 2-4 inches and to lower the water level 2-4 inches, depending on the heel and ride of the boat. Find a secure location (often the floor while on rough passages) and make certain the wires and compressor stay dry (mount above the aquarium). A few fish nets are required as well. There's nothing more fun than watching a young un' track minnows, crabs and shrimp.
Microscope. Rather nerdy, I guess, but I've always through the world right in front of us was fascinating. It's settled arguments re. whether black beach sand was charcoal, coal fines, blast furnace slag or volcanic in origin (yup, volcanic sand on the Chesapeake, who would have thought). We've watched blood coursing through the toes of a tiny pollywog. We've showed visitors that the tiny water fleas that were biting them were actually immature blue crabs. Real science? Not so much. Just fun stuff, although the possibilities abound.