Caladh Harbour is a favourite, and therefore frequently busy, anchorage just to the West as you leave the 'cans' going North toward Loch Riddon from the East Kyle of the Kyles of Bute. This link shows a nice aerial view of the harbour as well as the Ordnance Survey map; the latter being quite apt as the Harbour was reputedly used for landing craft and midget submarine training during World War 2.
I suspect that they may simply have used it as we do, as a base because it is probably too shallow and small for any real training (waits on the experts correcting him!). There' more on this on a fascinating website with superb information about wartime activities on Bute.
By far the easiest entry is from the South but some brave souls like to play with the tide and use the North entrance.
This pic shows the old light to the port side as you enter the harbour. Once inside try and pick a spot that does not block the entrance for others! Please!
Just be careful of the pile of rocks which emanate from about the centre of Eilean Dubh, and avoid dragging the moorings that are arranged on the West side of the quite small 'lagoon' that is the Harbour.
There are some nice photos and a bit of history about the Harbour on Ian Johnston's kayaker's page. Turns out that maybe Caladh Harbour is a bit of redundancy as 'Caladh' apparently means 'harbour'.
Linguistics aside, once you have the hook down be prepared for an idyllic calm (Southerlies permitting).
The estate surrounding the Harbour is kept impeccable and is very photogenic. The old boathouse and cranes are shown here.
The Harbour is a popular spot so you may either not like close company or simply not be able to get anchored. If so, you could try going North into Loch Riddon or a bit East to another nice, but seldom used, spot off the mainland.