There's very little to see here except for a carving lined walkway, two smaller temple pavilions, one completely demolished and the other standing precariously.
Nevertheless this is a peaceful site, under a forest of large trees squeezed between the Mekong and the paddy of nearby Ban Nakham Noi. Unless you're fascinated by this sort of history, you may well be disappointed by what you see.
Boatmen in Champasak will propose this trip to you there and back including waiting time and with a stop off at Ban Wat Luang Khao on the way if you want. The trip takes about an hour downstream and 1.5 hours back on a very narrow longtail -- don't move around too much.
Try going early or late to avoid the worst of the midday sun during the unsheltered trip. You can also reach Um Tomo by motorbike heading south on the main road to the 4000 Islands and the Cambodian border -- about 11 kilometres from the Champasak turnoff.
The turnoff is not clearly marked, but ask around, the villagers will know where you're heading. Then it's two to three kilometres of well-packed dirt roads to where there's a sign leading the way.
Note no food or drink is available at Um Tomo, but plenty of stalls are located at the turn off.