Getting a fibreglass pool 12x25 however it looks like (maybe it’s common with fibreglass) that this measurement is not swimming space but measurement from exterior shell wall so actual useable space is less. I am so disappointed. Wishing we had gone with vinyl. For vinyl pool I am guessing it is a true measurement of swimming space? We were told we’d need a retaining wall with vinyl but not with fibreglass so we were hoping to avoid retaining wall, not feel we sacrificed pool for maybe not that big a deal of having a retaining wall. Thoughts ? Losing sleep over this project.