You said you'd never heard it that way, I just wanted to clarify that I communicated the right pronunciation since "sewer" is a bit more drawn out than I meant to imply. All good
Drainage system = soo-er
Person who sews = soh-er
Exploring a place, with or without a guide = tohr
That's typically how I hear those pronounced. Idk, I get the sense that some think I'm trying to correct the OP when I'm just trying to figure out how the hell something is pronounced.
In most American dialects and some British dialects, "bore" and "tour" rhyme (called the "pour-poor merger"). But in some dialects it may rhyme with "sewer"/"two-er" or have the same sound as in "blue" or even as in "were".