This temple raised on platforms on the Beira Lake is probably the most beautiful place in the neighborhood -- it's a beautiful and unexpected piece of art that was designed by Geoffrey Bawa and funded by S. H. Moosajee in the 1970s.
The temple is divided into three raised platforms, connected by walkways - Bawa was apparently inspired by the temples of Anuradhapura with similar design. Monks are ordained here and rituals like Poya Karma (where monks state and apologize for wrongs done that month) are conducted here. The design flows out of that need. For certain sects the place of ordination needs to be surrounded on three sides by water and not be touching the ground, as this temple is built.
As a lay person visiting, you'll notice the temple's signature four-sided blue roof on the central platform, a structure shared by the 'Treasury of Truth' on the next platform. The central building has beautiful wooden paneling with gaps that provide a view onto the lake. The space contains several statues of the Buddha - it's quiet and warm, a good place for meditation. There are usually one or two caretakers here who'll help you if you have questions.
The space just outside this room is surrounded by seated Buddhas; some of them have their hands on their lap, while some show yogic gestures. The Treasury of Truth in the next platform is basically a space for Buddhist texts - it's usually locked up, probably only accessible by monks.
The next platform is the most beautiful - the bo tree in the center is flanked by statues of the Buddha, some very small and others gigantic - the biggest stands magnificently next to a stupa.
In the four corners of this platform are wooden paneled doors that open to images of Hindu mythology like this one of Ganesh.
Flags to carry wishes and ward off evil have been hung up by visitors on the trees on the periphery of this platform.
The Seema Malaka temple is an impressive space, meticulously maintained. When you're there, all you notice is the water, the sky and the calm under the bo tree. It's definitely one of the better temples in Colombo for its natural aesthetics, and a must-visit if you're at Beira Lake.
Remove your shoes before entering, obviously.