Doing the Abel Tasman Great walk is a wonderful thing to do. Marahau is going a good place to start / end your "Tasmanian" stay.
If you are not looking forward to this beautiful track, you can shorten your stay and use the water-taxis to take you to parts of the National Park, walk parts of the track you chose, enjoy the beaches and the turquoise waters, then take a water-taxi back to Marahau.
All boat skippers have witty personality, and can easily change drop offs if it is convenient enough.
I don't know if the track won't turn into a mud pool after heavy rains, but on good days, it is a family friendly track to do. You can even arrange with the water taxis to drop your bags off where you'll camp so you enjoy the walk without carrying your heavy bag.
The waters are so clear that it is a perfect place to rent a kayak when the sea is calm. You can get close to seals, get introduced to their newborn pups swimming not far from them. They are really friendly in water, but are more aggressive on land, as they are not as mobile as they can be when swiming.