I think of Smoo cave as just the main chamber where one could peer into the side passage on the right hand side out of which a stream issues. This side passage is the basis of the boat trip from an elevated walk way, followed by a simple walkable stream passage. (Or you can traverse along the side from before the issuing stream and climb over the rock bar.) You can then swim / traverse up to the base of the waterfall and the sump. You can also visit the top of the waterfall as it is on the other side of the road to the car park where the stream sinks. I doubt if it will be dry enough to climb up the waterfall but possibly you might be able to abseil down it. Also off the main chamber at the rear left is dig, a grovel which will probably be wet. You can abseil down through the hole in the roof of the main chamber. There is also a climb up the left hand wall but the black hole as seen from the ground goes no where. (I am unsure about anchors at all locations since it was 40 plus years ago when I did it. A 50m rope should be more than sufficient if you are prepared to rely upon the fence posts.)
Flooding is a problem of making way against the water flow rather than loss of air space, though I recall one visit when the air was thick with spray in the main chamber - a great sight. The water will be cold and I also recall getting tangled with fence wire in the stream way.
I have a feeling the boat is removed over winter to minimise damage to it so you would be on your own. The main chamber is simple, does not really need caving gear and is worth getting out of the car for. The stream way depends upon the stream flow and as mrodoc says obvious. Going at low water would be better, high tide with a northly gale will isolate the cave entrance.http://www.smoocavetours.com/other-caves-of-durness.html
lists other caves in the area but weather and water level will govern accessibility. I would only comment that it would be better if you have a competent buddy to instigate a call out from each location if needed.