First... at least 2 years before real changes and after that no one knows so all this is pure conjecture. But assuming no special transition arrangements are in place we can assume -
Brits... prospect of having to comply with the 90 day in 180 Schengen rules.
Brits will lose access to the European health insurance
card providing free health care
Unless special provisions are enacted it can be assumed that provisions applying to foreign boats at the moment will apply to Brits .... eg
New UK boats may be subject to EU VAT after 18 months in the EU irrespective of UK vat having been paid.
Existing UK boats that remain out of the EU for extended periods may lose their Vat paid in the EU status.
Depending on how the CE issue is treated you can expect this certification
to be changed or dropped which may make purchase
/ importation of US / Canadian boats easier in to the UK.
used European boats and importing them into the UK could be required to pay UK Vat.
British boats will probably be subject to customs
limits on booze and tobacco when returning to the UK.
It is unlikely that UK training schools will be able to issue CEVNI certifications for inland European waterways.
Issues for Non Brits.... a new place to reset the Vat clock on the boat which should be great for North American cruisers.
I am guessing there will be 1000s of other smaller issues but in time I suspect England
will start to feel like Canada
where the 500lb gorilla of a neighbor seems to set all the rules and has all the economic power.