For widebeam craft domestic sinks are no issue. For a narrowboat consider compact solutions from motorhome accessory suppliers in addition to reviewing an increasing range from traditional chandlers. It  is conventional to waste directly into the canal through a  reinforced flexible pipe and through hull spigot fitting. However, small domestic traps can be adapted to fit onto the reinforced pipework to prevent odour from canal water and gusts of wind blowing up the sink. A trap will require regular cleaning.

Likewise for bathrooms. Small domestic vanity sinks work well in a confined space, as do motorhome and chandlery retailed products. It’s great fun looking around for that perfect product.