Yes, I know, there is no sea in Chiang Rai, but there is a beach, a river beach. There are more and more places like this in Thailand. They mostly consit in the installation of small wood and bamboo huts at the edge of the river where you can sit on low tables and order food. Depending on the season, the stength of the currents and the color of the water often muddy in the rainy season, the inhabitants bathe there or at least play in the water there. It is a nice place to relax especially popular on the weekends and quite desert on weekdays especially in low season.
I recommend you to take the opportunity of going there to visit Rai Mae Fah Luang 800 meters away. This beautiful park has a large lotus lake, a museum and interesting old wooden houses.