Having just put my wooden boat into the water for the first time for 24 years I know about hanging head down in the bilges looking for leaks!

The first stage is to pump it all out and then watch carefully to see which section of the boat seems to be filling first. You may need to isolate sections temporarily with rags, mopping quickly to try and determine flow origin.
When you have narrowed it down, hopefully to a specific location rather than an entire seam, it is usually pretty easy to confirm, because you can mop the trickle and it will instantly reappear.

I'm assuming that a plywood boat is unlikely to take up over time, unlike traditional construction. So your options are either to pull the boat out and work on a repair or filling from the outside, now you know the precise location, or else to attempt some kind of temporary bodge while afloat.
If you can reach the spot, either from the dinghy or in the water, there are fillers that you can splodge over even under water. Stockholm Tar mixed with lime or polyfilla to the consistency of soft soap works quite well, and also tells you immediately whether you have hit the spot.
People say it is possible sometimes to do this with a brush on the end of a stick, but I'm a bit doubtfull there.

It may even be possible to stop or stem the flow from the inside, depending on what sort of leak it is. If coming up a crack or joint for example you could try wedging in a bit of rag, perhaps soaked in grease or tar or paint, using a screwdriver blade.