Three different water craft, gondolas, water taxis and water busses (vaporetto).
Gondolas are purely for pleasure rides, nobody uses them for getting from A to B, at least not since 1979 when Peggy Guggenheim died. Hers was the last privately owned gondola purely for transporting Peggy and her dogs (and lovers, one may assume). The gondolier will always help, and gondolas are flat bottomed so quite stable.
Water Taxis are a launch about 30 feet long. Generally there would be a couple of steps down into the water taxi, and at the airport you board a water taxi from a floating pontoon so the step is never too much.
Vaporetti. You board these from a floating pontoon, so the step down is never more than 8 inches or so, depending how loaded the vap is. The pontoons are stable, mostly, the exception being when the vap approaches and maybe have a heavy landing against the pontoon. When you go inside, the first row of seats are reserved for the elderly, pregnant or a bit infirm passengers.
Something to bear in mind, and to reassure yourself. Venice has the oldest median aged population in all of Europe, and so people are very used to helping others get around.