Why take the boat
A vehicle shows you the savannah. The boat shows you the other half of Akagera — the papyrus fringe, the floating vegetation and the shoreline, where a large share of the park's 500-plus bird species actually live. It is also the only way to be level with a hippo pod rather than looking down at one from a track a hundred metres away.
What you see
Hippo in numbers — Ihema holds one of the densest concentrations in Africa — and large crocodile hauled out along the banks. Birds are the real reason to bring a camera: African fish eagle, malachite kingfisher, African jacana walking the lily pads, open-billed and yellow-billed stork, herons, cormorants, blue-headed coucal and, with luck and a guide who knows where to look, the papyrus specialists including shoebill in the right conditions.
Buffalo, elephant and antelope come down to drink, so the shoreline often produces land game as well.
Practicalities
The standard trip is about an hour. Scheduled departures run at set times through the day and private boats can be booked, which is worth it for photography — you control where the boat sits relative to the light. Late afternoon is the strongest slot for both light and activity.
Where it fits
Take the boat in the late afternoon and follow it with an evening game drive, or run it the other way round with a dawn drive first. Either way, do both — they are different parks. See Akagera National Park for access and gates.