A common misconception is that the dolphins can be seen from Kratie town. They hang out at Kampi, some 20 kilometres north of Kratie along the river road.
Any motodop will take you there. You can also get there under your own steam -- by motorbike, bicycle or car hire ($5, $2, $12 per day respectively). It's a very straightforward route -- just keep riding straight and you'll reach it. Really, you can't miss it.
Once you're there, it costs $9 for one person to rent an entire boat or $7 per person with three or more people. You really don't want more than three in the boat unless you're on a tight budget, so if you're in a crowd, get a few boats -- the boatmen and the dolphins will love you for it. If you'd prefer not to get out onto the water at all, the dolphins can also be seen from the coastline -- though obviously they'll be pretty distant -- and you need to pay the admission fee regardless of whether you want a boat or not -- yes, that's right, $9 for a single traveller to sit on the riverbank. Though at least you'll be leaving the dolphins in peace.
From Kratie, it is around half an hour by motorbike (hour by bicycle) to and from Kampi, and we recommend going in the late afternoon when it's cooler and you can pair your trip with a stop at the nearby Sambok Mountain pagoda to watch the sunset. The staircase that leads up to the hilltop pagodas is flanked by a giant guardian statue, but the real attractions are the friendly nuns and the good views. At the apex you'll find two hilltops -- the smaller of the two is Phnom Bro (Brother Mountain) and the other, higher hill is Phnom Srei (Sister Mountain).