Ngaio There is something called the right of transit passage, which covers the Channel. With some exceptions, shipping from anywhere has a right to use it, even within what are known as UK waters. The UK would be contravening international law, if it were to send patrol boats offshore and interfere with passage.
Until the UK finally leaves the EU, the situation is easier for the UK because the border is effectively in France. As the UK is not in Schengen, there is still border control. Asylum seekers are prevented from boarding ferries and travelling to the UK. However, after the transition period who knows what will happen? If border control reverts to UK ports, asylum seekers will find it easier to reach British soil simply by boarding a ferry. I can't imagine France will want to stop people leaving the country.
And what about Ireland? Are people still going to be allowed to walk over the border and then claim asylum in Northern Ireland? Ireland isn't in Schengen, but the UK is still going to have to rely on Irish goodwill to stop people finding their way to the Irish coast and entering the UK.
The amount of irregular immigration pales into insignificance compared with the number of people who are entering the UK legitimately (and have entered since the outbreak started) and are still not being screened and/or quarantined.