Colombo not sure but Granbell has seaview tubs I would say. there was some hotels in Panadura, Kadawatha areas with plunge pools but not famous.
sorry for the typos was using the phone. Places I have visited with plunge pools / private pools
a. The Fortress (Koggala)
b. Wild Coast Tented Lodge (Yala)
c. Jetwing Kurulubedda
d. Malabar Hills (Weligama)
e. Water Garden (Sigiriya)
f. 98 Acres (Ella)
g. Sun Siam (Pasikuda)
h. Amari (Galle) - Now Radison Blu