http://www.philm.demon.co.uk/Miscellaneous/Sealion.htm
To get the first wave across, the Germans gathered barges and tugs, totally disrupting their trade in the Baltic. Eventually, 170 cargo ships, 1277 barges, and 471 tugs were gathered. These were, inevitably, bombed by the RAF (about 10% being sunk before they dispersed again).
The barges were mainly those designed for use on the Rhine, with a shallow freeboard. They sink in anything above Sea State 2. The wash from a fast-moving destroyer would swamp and sink the barge. (Correct: the RN could sink the lot without firing a shot).
..Finally, the barges were under-powered for open water operations, and required towing. The basic unit was a tug towing two barges, and travelling at 2-3 knots, in the Channel, which has tides of 5 knots. Given that the distance that the far left of the invasion had to cross, a minimum of 85 miles, the poor bloody soldiers would be wallowing for a minimum of 30 hours in an open boat, and expected to carry out an opposed amphibious landing at the end of it.
The most comical element of the plan, however, was that for manoeuvring the flotilla. The plan was that this huge mass of towed barges would proceed in column until reaching a point ten miles from the landing beach, then wheel and steer parallel to the coast. When this was complete, the vessels would make a 90 degree turn at the same time, and advance in line towards the coast. This was to be carried out at night, and controlled and co-ordinated by loud hailers.
There had been no chance to practise the operation, and there was less than one skilled sailor per vessel.
If this seems to be a nightmare scenario, and a recipe for disaster, it is as nothing compared to other elements.
...One single main exercise was carried out, just off Boulogne. Fifty vessels were used, and to enable the observers to actually observe, the exercise was carried out in broad daylight. (The real thing was due to take place at night/dawn, remember).
The vessels marshalled about a mile out to sea, and cruised parallel to the coast. The armada turned towards the coast (one barge capsizing, and another losing its tow) and approached and landed. The barges opened, and soldiers swarmed ashore.
However, it was noted that the masters of the boats let the intervals between the vessels become wider and wider, because they were scared of collisions. Half the barges failed to get their troops ashore within an hour of the first troops, and over 10% failed to reach the shore at all.
The troops in the barges managed to impede the sailors in a remarkable manner - in one case, a barge overturned because the troops rushed to one side when another barge "came too close".
Several barges grounded broadside on, preventing the ramp from being lowered.
In this exercise, carried out in good visibility, with no enemy, in good weather, after travelling only a short distance, with no navigation hazards or beach defences, less than half the troops were got ashore where they could have done what they were supposed to do.
The exercise was officially judged to have been a "great success".