Removing drains is gloriously variable - different surgeons, different patients, different preferences. Mine came out after 3 days, fluid had stopped draining well and truly. I too showered day after my mastectomy, still with drains in. Trick is to get as much drained as possible without risking introduced infection. I did get a seroma but I don't think the two are necessarily connected. I healed really well, no pain. Just my body kept producing fluid, to help healing but over the top. Trust your team, they are making the best judgement they can. Best wishes.