Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
Water boatmen are very common in trout streams, but they aren't an important prey for most trout most of the time. Occasionally they are the important prey and trout feed on them selectively. This is especially likely in weedy mountain lakes and spring ponds.
Where & when
Time of year : Mating in early spring
Preferred waters: Most abundant in slow or still weedy waters
Boatmen are available year-round, but they have mating flights in the spring.
Time of day: Afternoon
I had never read of this behavior before I saw it for myself. Boatmen climb out of the water in early spring (April where I watched them in northern Wisconsin) and are easily spotted on the remaining snow banks next to the river. The adults fly around over the river, sometimes quite high, and eventually splash down onto the surface, where they make a few kicks to break the film and then disappear. I presume this was their egg-laying behavior, and they did it in such numbers that trout probably fed well on them.
This is based on my one encounter, so there may be more variation in their behavior and I may have misread something.