Downhill skiing is an excellent cross for a shorter distance faster, explosive rider, while X-country for a longer distance - except that will lead to a larger upper body, usually a detriment to long rides (cause of hills).
So I'd bet on the downhill skier to be faster than the x-country skier in anything under 30 min and likely an hour (race). But the X-country skier may have the lower resting HR and measure fitter in the lab.

Rowing (crew) also is a great cross. It tends to be a pretty short event, and is less explosive, but they deal with lactic acid build-up and the limits of what the body can do as far as power output.

All sports have their own nuances and technique that makes things "easier". A less fit experienced rower is going to row faster than the fitter newbie.
Swimming is a great cardio, cross but not so much useful leg work. Even kicking is more aerobic. But doing that 1 day a week as a mental break can be good.