I'm going to address your concern in two parts: the practical health part, and - I suspect more importantly - the issue of trust in your relationship.
You seem very knowledgeable, and on top of the rules and guidance - as someone in your situation needs to be. It may be worth thinking about how to limit any possible risk from your husband's behaviour and choices, and the most important piece of that is if you, your husband and his friend are all vaccinated, then the risk is low.
You may consider also asking him to wear a mask when on the boat with his friend, and asking him to get regular tests.
Worst case, you may need to isolate using masks etc, inside your home bubble. I could imagine he may not like this, but that may just be required as a result of his actions, and it also leads to the second part of my response.
The relationship.
Trust is fundamental in relationships, and as such, I can understand that you're feeling very upset. I would invite you to consider this differently though, if it is unusual for him to breach your trust like this, or not.
If this is very much a one-off, then I think it's important to talk with him about why it felt so important to him do what he did - and apply one of the important rules of thumb of lockdowns and the pandemic - at least with relationships - give him some slack to get it wrong. None of us are at our best at the moment, and if he is "clear" of any infection, then no harm done.
If, on the other hand, this is a pattern you can identify - one of him not listening or being able to work with your stated boundaries and limits - then I'd feel more concerned. In that case it's probably important to spell this out, and challenge him - as gently as you can manage - to listen to how harmful breaches of trust like this are to your relationship.
Ultimately though, finding a way through this is going to require both people being willing to talk to each other, share their point of view and try to reach a mutual understanding.
If you feel you can do that with him, then great.
If not, then it may worth considering accessing some additional help from a relationship counsellor to figure out what goes wrong at these times - and how to mend the broken trust.
In the meantime, when you do talk to him about it, it's okay to be angry, but it's also important to try to be effective. Try to talk to him using "I statements" - talk about how you feel, and why it's important to you - and stay away from "you statements" - "You should know better", You make me…" etc.
Overall though, regardless of what you end up doing, it's okay to have the feelings you have. Out of frustration - or out and out denial - it can be too easy to simply throw aside the rules and decide we're going to do what we want in lockdown. But even though most of us now hate them we have these rules for a reason, and important to remember we won't need them forever.