Hi, this is just outside Hai Ba Trung ( I think it's Dong Da ), but the Kim Lien hotel has a small-ish pool that is open to the public for (iirc) 20,000 D. It's a big government hotel near the junction of Dai Co Viet and Le Duan/Giai Phong.

HTH

There is another one called Thanh Nien (Youth) swimming pool in Tang Bat Ho str.
When I went there it was often very crowded and the water was a bit dirty, but that was like 6-7 years ago though.

swimming pool in Bach Khoa (inside Hanoi University of Technology)
Tel: 8692247

four seasons swimming pool
Add: 4 Tran Hung Dao
Tel: 933-0796

four seasons swimming pool
Add: near by Bia Hoi - A15, Ngoc Khanh str

Average price: 20.000 - 25.000vnd

Try the swimming pool at Melia Hotel on Ly
Thuong Kiet ( about one block south(?) of Hai Ba Trung Street. The pools are a nice size and the setting quiet. I don't recall if the swimming area is open to the general public, but it is worth an inquiry at the lobby desk.