Edit: Hadn't realised this was so long until posted... no matter might be something for you or others in the waffle...
Steve this is a very personal choice re solitude vs facilities and boating excitement vs a few decent family friendly days out.
I forget what is the makeup of your family for sibbing? What else do they like to do?
We've been taking a cottage in Scotland around many areas except the very far north for some 35yrs+ now.... and it's been boating based for most of the past decade... SIB and or 4m trailed hard boat. So we've pretty well refined what suits us...
We want the cottage to be just yards from a slipway or easy beach launch... no more than a 2min trundle on transom wheels. Minimal tidal range helps but in any case if slipway it needs to be all states of the tide or if beach no long transits across sand/mud/stones.
A sandy beach if not adj must be within reasonable reach by boat and the boating area should allow a reasonable choice of trips without having to operate in areas of strong currents/overfalls etc.
The launch site must be sheltered and useable even in less than ideal weather.. nothing worse in a SIB than a difficult transit home then in the last yards being faced with breaking waves on a beach/slipway.
Cottage location is often a balance of privacy (if you value that) and amenities. We are wary of one so remote where it's a 40min round trip for a pint of milk so ideally like to have a village shop within easy walking and a weekly shop sized place no more than 20mins away.
Fuel location is essential as we find it impossible to carry a week's worth... also an eatery you can quickly walk to is a huge benefit.
So much easier now with Google maps and online info... in the old days the cottage brochure came by post and I had to go in the library at lunch times to try and make sense of the area from Ordnance Survey maps and work from the Le Routiers book for decent eateries.
Anyway it might not suit you and doesn't tick all my above boxes but here's just one I'll chuck at you...
Ellary Estate cottages situated on Loch Caolisport in Knapdale. Six cottages of differing sizes/prices set on a fairly remote private estate on a no through road. All having access to a very sheltered small sandy bay ideal for operating smallcraft. There is also their tiny private harbour if you want to stay afloat.
We stayed there several times and found it a great base for boating. Over several weeks we only had one day the loch was too rough to launch and you have a great "no road access" secluded beach towards the head of the loch to visit/picnic if you want to avoid nosing out into the sound of Jura.
On any reasonable day you can swing round into the adj Loch Sween which gives you the fantastic Kilmory Bay beach... the beach and café eatery at the castle then Tayvallich at the very top with pontoon, small shop, café eatery, pub etc.
You could fill a week with family outings to the above places without any great risk of scaring them.
The downside is that the nearest shop of any size by road is at Ardrishaig which is 11mls down a mostly single track road.... or even more variety at Lochgilphead some 2mls further on. But we managed shopping on arrival Sat and lasted until Wed before leaving the place for our only food/fuel outing and didn't shop again until we left. No nearby eateries at all though.
Ellary Holiday Cottages
Three pics... Our hard boat and Zodiac at the cottage's launch bay. Long view of secluded beach at head of Caolisport and our hard boat on that beach.