In the Solent, offshore from Daedalus airfield, Lee on Solent.

I think it's carrying out survey work related to the planned IFA2 electricity interconnect with France http://www.ifa2interconnector.com/.

The IFA2 cable is intended to land at Daedalus, with an onshore DC/AC converter station feeding another offshore cable to Brownwich/Chilling further west, where it will connect with the National Grid.

Solent Guardian seems to have been surveying the planned Daedalus-Chilling cable route in recent days.