There are a number of excellent tidal lagoons along the Abel Tasman coastline (e.g. Torrent Bay Lagoon, Frenchman's Bay Lagoon, Falls River, Bark Bay, Awaroa Inlet). Generally these can be explored by sea kayak 2 hours either side of high tide. There is insufficient water to kayak in the lagoons outside these times.
If walking the coastal track, you will need to time your walk with the tidal crossings.
Do not attempt to cross the tidal estuaries at high tide! The tides rise and fall twice a day and need to be considered with regards to the scheduling of your walk or sea kayak trip. Once you have decided when you will be in the park, work out the high and low tide times. (See below). There is one place that you can only cross around low tide: Awaroa Estuary. Torrent Bay Estuary and Bark Bay Estuary have an alternative high tide route, but these are longer and slower than the low tide crossing routes. The exact timing of your window for crossing will depend on which place, phase of the moon, weather conditions.
2 hours either side of low tide is fine for Awaroa Estuary, and just take the high tide route around the others if it looks too deep. The 2 hour rule offers the best margin of safety and comfort.
To determine tide times for a given date, use this site.