My first thought is that your gain is too high. It appears as though your reflections are "smearing" so that targets on both sides of the channel are blending together, causing a lack of definition and the apparent targets that are across your path ahead of you.
At that range, it's unlikely that you'll be able to see individual boats. But you should be able to see the clear channel ahead of you, with an irregular or "bumpy" edge on each side. Sea/Rain clutter are usually inconsequential at that range, and should be on the low side. My first step would be to try to remain in the same spot, and play with your gain settings until you see something that mirrors what you're seeing visually. Read up on your manual (available online if you don't have a copy) on how to tune the radar. Beware that a lot of the old Furuno manuals tend to be written in "Engrish" and can be a bit tough to follow. Get the model number and check out YouTube, there may be some helpful hints there.
Also, take note of how the radar dome is mounted on your hardtop. Unless it has a downward angle towards the bow, at very low ranges there will be a blind spot around the vessel where the radio waves will shoot over anything in close proximity.
At the end of the day, remember also that that's a pretty tight passage for a small radar unit. The closer targets are together, the greater their tendency to blend or smear together, looking like one target. That's a situation for very minimal power. The old Furuno units are good radars, but you're not going to match the definition of the new high definition/variable pulse/doppler units. But by playing with it a bit, you should be able to get something that is navigationally useful.
