Doppler systems tend to unlock over calm water.
Yes, and coupled with a single-channel analogue AFCS, it always made for exciting IMC or night SAR letdowns to the hover in glassy conditions. So much easier now with inertial/GPS systems.
We used to get the Wessex rad alt lock onto the sea bed in Cyprus but that was over shallow water - down to about 50' ISTR - can't imagine that was an issue in the Ionian Sea.