So, first of all, I started off this discussion talking about the effects of armor on swimming. Many good points were made about armor not being the only consideration. So, I've changed the suggested advice/ruling I intend to make. As it stands currently:
"The published rules for swimming may feel inadequate in situations that may occur during these encounters. These guidelines are to help DM’s make rulings if these situations arise.
Characters needing to swim in the Silverthorne can do so with a DC 12 Strength (Athletics) check. In some situations, these checks may be modified, characters that need to make swim checks that can be considered at disadvantage for these checks may include those; wearing armor that impose disadvantage to stealth checks, carrying objects weighing more than 50% of their carrying capacity, holding a shield, or wearing things such as full robes."
So you know the setting/assumptions about when this ruling would be appropriate; The characters are on a boat, in combat, on a mid-sized river with a current of about 2mph. They probably are not carrying all their equipment as it is likely to be stowed on the boat somewhere.
I need to add a sentence or two about asphyxiation, because as made obvious by the discussion here, many people assume a single failed save would indicate death or drowning. That is not the case per the rules or this suggested ruling.
The other effects of water, such as destroying a spell book, spell components, or doing other things that water does, are not addressed. And though I could address them, I don't see a reason to do so. I've never seen a published adventure make such recommendations nor have I ever seen any rulebook make such rules. Apparently, for most gamers, such considerations are not part of the fun or are left totally up to the DM to institute as desired..
So, understanding the situation the characters are in, understanding the intent of the ruling (to increase fun by adding diverse considerations), keeping the ruling simple to use, and adding an acceptable level of verisimilitude;
How does the suggested ruling sound?