In/on for "into/onto" in colloquial and not so formal AmE

In and into are not, strictly speaking, grammatically interchangeable in formal writing. On and onto have a little more leeway, but are also not always interchangeable.

But, colloquially we often use in for into.

It is perfectly acceptable to say go in the pool, jump in the pool, etc. But, as you correctly stated above, that introduces ambiguity:

I am going around in the pool. -- Unlikely to be the intended meaning, but it could be.
I am going into the pool. -- The more likely intended meaning.
I am urinating in the pool -- This might also be a likely interpretation from the colloquial usage of go to mean urinate (Thanks Digital Chris!)

I am jumping around inside of the pool. -- Again, unlikely from context.
I am jumping into the pool. Most likely scenario.

You can get on the bus, and you can get in the bus. But, they have subtly different meanings:

Get on the bus - Board the bus with the intention of riding.
Get onto the bus - Is awkward since it implies climb on top of. Better to say get on to the bus.

Get in the bus - Get inside the bus for whatever reason.
Get into the bus - Get inside the bus for whatever reason.

But, note that we don't get on the car. We do get in or into the car, though.

For whatever reason buses, trains, planes, and other large conveyances have a notion of being board-able (employing the preposition on), but cars and vans generally do not. (This may be convention.)

Compare this to a bicycle, motorcycle, etc.. You get on a bicycle, but this is the physical sense of being on top of it.

Languages are always translated into one another. Never in. But, you can speak in a language.

Some other cases where on and onto are not interchangeable:

Turn on the lights.
Turn onto the lights - You'd probably never say this. It implies that you physically are turning yourself onto some lights.

But, they are interchangeable when on carries the sense of on top of.

Put that on the counter.
Put that onto the counter.

In many cases, in is used for into, but not always. We get in the car, go in the water, get in the driver's seat, go in the store. We get in the shower, we get on the bus, but we do not translate one language in another.

It somewhat depends on the verb as well as the activity. We tend to climb onto, squeeze into, drive into, latch onto, and others. In and on are not freely interchangeable for into and onto.