It's not just deep water that is risky!
The water depth at Botesdale is currently no more than 1.6m, or 4feet. That is water depth, but the silt itself can be a couple of feet deep. The causeway itself is wadeable, you can walk across it, it is a proper stone causeway built around the 15 or 1600's, a stone pathway built across the marshland to the manor house before Lancelot Capability Brown created the lake.
Step off the causeway towards the shallower (east) end and most of it will go to no deeper than thigh depth, however carp have fed up and created holes that go chest deep.
Step off the causeway to the dam end and you will be going into water chest deep, but with around 1-1.3mm or 3foot of silt you can sink in. Put a marker float out and if the float comes up you will get 3-4feet of water, the lead sits on it, as @greekskii says, it has a hard crust, except, again where carp have fed and it is softer.
There is also a stream bed running along the lake on the house side, which is a metre of silt.