Does Namor only become stronger in ocean water? Or does fresh water (i.e. lakes, rivers, etc.) have the same effect? Just curious...I'm not very familar with the Namor's history....

Any water...as long as its not heavily polluted

No.

The water can be from any source.

Water in any form will boost his strength and stamina. He's used the X-Mansions pool to heal himself within seconds after being gutted by Wolverine. He's also fought during a rainstorm and his strength was nearly doubled.

His strength in any situation is heavily underrated, but yeah, any water. Although it's stated that under the ocean his strength is "unmatched" (thank you Marvel for that horrendous Thing feat recently). Namor's high end feats put him far above characters like Colossus and Thing, yet still below Hulk/Herc/Thor level. Like Aquaman.