Never heard of anybody swimming across the [Danshui] river successfully but we did swim often to the delta, which was shown on the US military map of WWII but since disappeared.
[The map site is still active, click the above link, see also below:]
(Map of Danshui 1944-5. Locations are in Japanese pronunciation and military targets are in English. Click to enlarge.)
The best time to get to the delta is at the low tide and the best spot is behind the public market. By the way, living in Tamsui we need not to have a tide table since noontime is always high tide upon the first and fifteenth of the lunar months. The salt-water fish arrived with the tide and the biting only lasts about half an hour. For catching the fresh water fish, like striped bass, you have to go upstream as far as to Kan-Tou 関渡, at the red arch bridge designed by Prof. T.Y. Lin 林同棪of the University of California at Berkeley.
The delta, we call 浮線 (Pu-Swaah, close enough, EyeDoc?) is merely 100 yards away at low tide but the river flow is fast, so aim upstream diagonally and start swimming with full speed, hope you will reach the other side within several hundred yards downstream. There is nothing on the delta except a single tomb, built by sailors for an orphan. Sailors adopted him but he drowned in the river later.
Navy has a small ship similar to U-Boat parked behind the Police Department, probably 100 yards from the shore. Swimming over and board on the ship time to time, soon we make friend with the sailors. We learn later that the boat is there for a sole purpose, waiting, in case high-ranking offices need fast escape from island. The duty of the sailors is simple just turn on the engine key once every day and make sure the boat will start. Not a high-ranking officer but a VIP, madam Chiang visits Tamsui often, especially near the sunset, probably for enjoying the famous scene.
Ah that clam, it is Hama-Guri in Japanese, (濱栗) means the chestnut of the beach [see picture on the upper left]. There is a native kind; sharp edged more like a wedge, however, the familiar kind is new immigrant from Japan and it can grow up to 4 inches size though they are harvested prematurely nowadays. The common method of clamming is using a rake on the sandy beach that showed up at low tide. For the big ones, we call 老蛤 (Rao-Gyou, close enough, EyeDoc?) we have to dive to get them. Once upon a time, at the lowest tide of the year, I dive into the ship channel, probably ten feet deep and grope with both hands. There are many gravel-sized clams partially embedded into the riverbed every few inches apart. They are the big ones. I have collected almost 20 pounds of big clam that day.
Boil and season with ginger and onion is the best way to make clam soup. For the big ones, we can simply bake it on the fire. It is ready to eat when the shell pops open. Japanese like to add few drops of Sake but I do not. I have discovered a big worm, couple of inches long inside the clam once. It must be a parasite, I guess.
[Note: The Romanized Taiwanese names are perfect - EyeDoc.]