I'm dealing with a primitive tribe (of a species that can breathe underwater) that lives deep under the sea, and has little in the way of human technology. Resources include things naturally found in the sea, and things salvaged from shipwrecks and human waste (i.e. plastics) (though they do not know how to use human technology).
While the species can breathe underwater, they can't poke any part of their bodies above water - to do so would violate some kind of sacred law. Getting close to the surface is frowned upon culturally. They typically stay far enough away from the surface that they don't receive a lot of light.
That said, they need to be able to track lunar phases, tides, and passage of day and night for their religious calendar system. Without looking directly at the moon or venturing close to the surface/shore, what would be an effective way to measure these cycles?
EDIT: In light of the first answer, and to open up any further ideas that they might give you, I should mention the species possesses natural sonar, infrared vision, and a shark-like "electroception" sense. They're also likely skilled at gauging water pressure to avoid "the bends", as divers call it.