I read an interesting bit of analysis yesterday, (apologies because it was in a newsfeed and I can't find it again), which basically said "small boats" are priced in to future votes and the Tories will get little mileage out of it.

The basic thrust is that:

a) Immigration concerns, which peaked at about almost half of voters before Brexit is now considered a vote switching issue in only one in 5 people.
b) People who are considering it an issue worthy of switching a vote are more than likely to switch it away from the Tories because every time "small boats" comes up as an issue it is linked with the party that has been in office for a decade and a bit so trust has gone.
c) If the latest "noise making gimmick" (my words) does not resonate with the 20%. who are overwhelmingly in the Tory camp anyway, then they will still blame the Tories and leap to Farage or elsewhere whilst little or no votes will be attracted by the policy to compensate.

Still, if after years of palpable maladministration you have a shit hand then a bit of bluff and bluster is all you have left I suppose.