I am writing a story set in the 17th century and have read accounts of such early submarines, however, not much detail is available. If some kid of weights/plummets are attached to the sides of the boat to submerge and the crew is using sealed oarlocks to move it forward and breathing through breathing tubes, can they walk the river floor, say knee deep, without the water coming in? Just trying to find the most plausible and not scientifically offensive version...Thank you!
The Mythbusters have tested this one. You might want to watch the episode :).
Myth: You can turn a row boat upside town and use it as a makeshift sub
This myth was used in the movie Pirates of the Caribbean -- Captain Jack Sparrow and Will Turner find a row boat on the beach and sneak out into the water with it over their heads. As they enter deeper water, they are able to use it as a makeshift sub to walk along the bottom of the water, with the boat providing air for them to breath.
Jamie and Adam walked out into a swimming pool in Oakland pool with the upside-down row boat over the head. They made their way into deeper waters and quickly found themselves hovering -- they were too buoyant to walk along the bottom. They added more 'pirate' gear to their bodies (e.g. cutlasses and pistols) to see if the additional gear would push them to the bottom. The second run had the exact same results.
They decided to approach the problem from the other direction and see if it was even possible to hold onto the boat while it was at the bottom. They put on scuba gear and sunk the boat to the bottom. 200 lbs of barbells were used to keep the boat on the bottom and another 500 lbs of barbells was attached to Jamie and Adam to keep them anchored. As the boat caught the air bubbles from Jamie and Adam's tanks, it quickly became too much for them to hold onto and floated to the top. Clearly it wasn't really possible for two people to even hold the boat below water even while anchored.
Finally, they put the boat right-side up and started filling it with water and weights until it started to sink. It took 2000 lbs to sink the boat, which means that you 2000 lbs of force to hold onto the boat.
They showed one last sequence to demonstrate how they could have shot the scene in Pirates of the Caribbean: they could have drilled some holes in the boat in order to walk with the boat underwater.
I'd say it is plausible if you got the weights just right. It sounds tricky in the easiest of circumstances though - no chance you would be able to pull this off without prior attempts or scuba gear. However this is difficult to judge without having seen the episode myself.
Sorry, but plausible is not likely. Your description has two problems. The big one is the breathing tubes. Once you get down more than a couple of feet, your lungs simply aren't strong enough to inhale against the pressure trying to force the air in them into the tube. And the same pressure will cause all the air in the boat to escape through the tube if you take your mouth off it. If there is no breathing tube, the air pocket in the boat will get smaller the deeper you go. But it's actually (in principle) possible - if you can adjust the size of the trapped air bubble. The way to size it is to use a boat that rides very low in the water. Or, to put it another way, the heavier the boat, the bigger the trapped air bubble can be. The Mythbusters segment was good, but they used a light boat and didn't fiddle with the bubble size. You might have your hero start with an overturned, no-bubble boat, then bring down overturned empty buckets of air to fill the hull with air one bucket at a time. Also, the boat will be unwieldy, and depth control will be a nightmare. If you start at some depth (say 6 feet down), with the boat neutrally buoyant, if it rises just a little the pressure will drop, the bubble will get bigger and therefor more buoyant, which will pull the boat up -and you see where that will go.
I must say,the way mythbusters handled this myth bugs me intensely. Yes it's true it wont work the way it was shown in the movie, but it definitely can be done. Just weight the upturned boat until it has a slight positive buoyancy.Then it's easy for the guys to hold it down. Job Done