Find a dog sitter who will live with them on the boat while you're away. That way they keep to their usual routine and they're surrounded by familiar smells and belongings so get less stressed.
When I've done dog sitting it's always in the owner's home to minimise stress on the dogs. I always meet the dogs and owner on neutral territory a few days or weeks beforehand so the owner can tell if the dogs like me (and If they like me too, of course). A day or two before they leave, I'll be invited round to the home so the dogs not only remember me from a low stress environment previously, but also get used to seeing me in their territory with their owners relaxed in my company. Then when it comes to the day the owners leave, it's not a stressful surprise for the dog when I turn up at the house or boat or stay overnight. I feed the dogs and walk them at the same times their owner does. If they're used to the radio or tele being on at a certain time of day, I ensure that happens too. And theres lots if playtime to keep their minds active. The result is happy doggies who aren't pining or chewing furniture or using the boat as a toilet. It's particularly good for aging dogs, or puppies or boisterous dogs who agency staff refuse to look after or just ignore because they're seen as too much trouble. Poor puppers!
If it's a neighbour who is only going to be away a few hours, then the dogs already know me and if the owner prefers, I can pop on board the boat a few times a day and take them out for a wee and a stroll, instead of spending all day with them.
I've yet to find any dog care agency staff who are prepared to really spend time with the dogs and keep them to their usual routine. Most seem to want to do the bare minimum, won't clean up after dogs who poop accidentally indoors, or they have multiple animals to get round in a day.
If I were you, I would try to arrange well in advance to moor near someone you know well who can be trusted to look after the dogs while you're away - or meet and book someone to come over. Don't be concerned about the boat sinking. But do make sure anyone looking after them isn't freaked out by boats or gets the heebeejeebees every time it rocks a bit, because their nervousness will transfer to the dogs. And someone used to living in a house might do something stupid like leave the water tap dribbling or something electrical switched on, or lights on, draining your batteries. Or they might struggle to lock up hatches, complain about tricky towpath access or whatever. If they're not a boater, give them a bit of basic boat life training. Make sure you're moored somewhere secure so your pins can't pull out and scrotes won't climb on your boat and stress out the dogs. And as said above, definitely don't leave your dogs alone in the summer because boat interiors can get very very hot, even with windows left open.
Train a couple of friends or family members on being boat-sitter dog-sitters, then the dogs get to know them well and you'll have more flexibility around when someone is available to help out πΆπ