A shower drains because the floor is laid to fall toward the waste. If the fall is insufficient or uneven, water sits, and no amount of sealing fixes it afterwards.
Waterproofing sits under the tiling and is what actually keeps water out of the structure. It is a separate trade from plumbing but the two have to coordinate, because penetrations for wastes and taps are exactly where failures occur.
Moving a toilet is the change people most often underestimate. It is not just the pan position; the waste has to reach the existing drainage at an adequate grade, and how far it can move depends on where that drainage runs. That is worth establishing at design stage rather than mid build.
If a hot water system is leaking, making unusual noises, or delivering water hot enough to scald, switch it off at the isolation point if you can do so safely and call a licensed plumber. Tempering valves and pressure relief valves are not do it yourself items, and a failed relief valve can be dangerous.
Water and electricity together are a serious hazard. If a leak is running near a switchboard, a power point, downlights or an electric hot water unit, switch off at the mains only if the switchboard itself is dry and safe to reach, then keep everyone clear and call a licensed plumber. An electrician may also be needed.