Moose are among the very few deer that feed underwater. They wade into lakes, oxbows and beaver ponds, then put their heads and often their whole bodies under to pull up water lilies, pondweed and milfoil, surfacing with dripping mouthfuls.
The reason is chemistry rather than calories. Terrestrial browse in the boreal forest is very poor in sodium, while submerged aquatic plants concentrate it, often by orders of magnitude per gram of dry matter. A few weeks of pond feeding in early summer can cover a large share of an animal's annual sodium requirement, which is why moose return to the same wetlands year after year.
Field accounts describe dives of several metres and submersions approaching a minute, though these figures come from observation rather than instrumented study. What is genuinely unsettled is how much sodium a moose can bank in the rumen over winter, and how the drying or draining of ponds shifts that budget.