"Visit Scotland" (the Scottish Tourist Board) asserts that Scotland has "the best sea kayaking in Europe". Their website has a lot of background, some trip suggestions and good general advice, including a list of "operators". (Outfitters / guides to you and me). Some (but not all) of these offer kayak hire but you'll have to make your own enquiry as the site doesn't give that info.
Adventure Hebrides - half day and day trips run out of Stornoway in the Outer Hebrides
Applecross Mountain & Sea Guides provide sea kayaking expeditions and courses for beginner to advanced intermediate paddlers. They also run expeditions to the Arctic Lofoten Islands, Orca whale paddling in Canada and the Dalmatian islands of Croatia. Sea kayak and kit hire available all year
Arisaig Sea Kayak Centre run guided trips and hire out sea kayaks, canoes and sit on tops. Based at the Arisaig hotel in Arisaig village.
Clearwater Paddling are based on Barra in the Hebrides, but offer trips in Norway, Oman and Trinidad as well as their Hebridean ones.
Explore Highland are Inverness based and offer a range of courses, holidays and other activities, and also have a range of sea boats for hire.
Glenmore Lodge offers a variety of courses and trips, including sea kayaking.
Glenuig Inn offer a variety of courses and trips, and have Tiderace kayaks for hire.
"In Your Element" Formerly "Boots n Paddles" offer 'amazing' outdoor experiences across Scotland, including canoe & kayak hire and shuttle services
Island Cruising / Cuma specialise in trips to St Kilda and some of the following islands en route, Scarp, Taransay, Monach Isles and the Flannan Isles. As well as kayaking specific trips, they also operate as a dive charter, bird watching trips and almost anything else you'd like.
Kari-Tek, based in Ayrshire, will hire Nigel Dennis (SKUK) and Rockpool boats - they also have a limited range of roof racks for hire, so you could fly to Glasgow or Prestwick, hire a car (check the appropriate type with Kari-Tek first), pop down to Ayrshire (under an hours drive) and you're then positioned to take your hired boat away! Nice lateral thinking. With the Clyde on the doorstep, and easy access to road and ferry links to the West Coast, this has got to be worth considering if you're planning on visiting Scotland.
Kayak Scotland is based in Aberdeenshire in the North East of Scotland and offer a variety of day trips around the North East coastline and also offer multi day trips at various locations throughout Scotland including the West Coast.
Lochaline Dive Centre in the Sound of Mull offer accomodation and can arrange use of their "hard boats" - something which may be of interest to a sea paddling group looking for a base in the area.
Otter adventures provide bespoke sea kayak and open canoe tours around the West Highland Peninsulas (Ardnamurchan, Morven, Ardgour, Moidart)
Rowland Woollven is a British Canoe Union Level 5 Sea Kayak Coach specialising in individual and small group sea kayak coaching and guided tours in the Oban area and on the west coast of Scotland.
Rockhopper Scotland say they offer "high quality guided sea kayak trips based from Fort William" and also provide a hire service with pick up and drop off available.
Sea Freedom Kayak Company operate near Oban and "run trips and sessions for individuals and small groups". They also offer a number of P&H Capellas for hire to "suitably qualified people".
Sea Kayak Alice - Alice McInnes runs a range of trips in Scottish waters, including "women only" trips.
Sea Kayak Argyll - offer sea kayak guided journeys and coaching sessions on the Argyll coastline, including guided journeys along the 150km Argyll Sea Kayak Trail.
Sea Kayak Oban - a superb dedicated sea kayaking retailer in Oban. Boats available for hire.
Sea Kayak Plockton - offer a range of courses and trips based out of Plockton, including "women only" courses.
Sea Kayak Scotland are offering courses and trips "on the West Coast" - they also seem to have boats for hire - based on the Isle of Seil between Oban and Lochgilphead.
Sea to Skye Xperience offer sea kayaking and walking activities in Skye and Lochalsh in the North West Highlands.
Skyak on Skye. Gordon Brown's centre, in a superb part of the world. Trips and coaching.
Snowgoose are based near Fort William and offer sea kayaking as well as a range of other outdoor activities. They have sea kayaks for hire.
Summer Isles.com appear to offer sea kayak hire - based near Ullapool.
Tiso have some sea boats for hire. Shops in Glasgow, Perth, Edinburgh, Stirling, Inverness.
Unexplored Scotland (formerly Highland Ascent) run multi day expeditions and courses in the Highalands
Visit Scotland (the Scottish Tourist Board) have links to some of the centres in Scotland
WhiteWave on Skye offer a variety of activities, including sea-kayking.
Wilderness Scotland offer "small group" adventure holidays in Scotland, including sea kayaking.
Zoe Newsam Guiding - Zoe offers trips in a variety of locations in Scotland.