Johno will know better than I, but I didn't find the river a problem. The depth from the mouth of the river, where it runs into the harbour upto the 2nd road bridge where we turned around didn't drop below 1.2 mtr. Outside of the marked channel, the harbour from Mudeford to the the mouth of the river was really shallow in places. At 1.12 in the video the depth was only 0.2 mtr and it was 3 hours before low tide.
As Johno says, it can be tricky launching and recovering when the tide is running and even more so when it's windy as well, but providing you make allowance for it it's not a problem.
Towards the end of the video you can see a black rib which had just launched, he appeared to use shallow drive for a good 150 mtrs as they went towards Christchurch.
It's well worth a visit when the conditions out of the harbour are a bit iffy, the only downside is a lack of places to stop off and/or a visitor pontoon at Christchurch.
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