I've been trying to work out what you're trying to do here and I confess I'm still not completely sure. What I believe is that you have a raspberry pi with one interface as a client of a raymarine fishfinder (marketed as "dragonfly" in english speaking countries?) and a tablet connected to the other (wlan1). The fishfinder does not output NMEA data, so kplex would not forward any of those data. Presumably the data is broadcast over UDP by the fishfinder, you are collecting that and forwarding it from wlan0 to wlan1 using socat. So far so good?
Your kplex configuration references a USB GPS which I don't see you say is working. Your second message talks about a GPS connected via the pi uart which presumably does work. If you wanted to use that GPS and broadcast the data to port 2000 on wlan1 the configuration would look like this:
[serial]
direction=in
filename=/dev/ttyAMA0
baud=9600
[udp]
direction=out
device=wlan1
port=2000
For an output udp connection if you don't supply an address, kplex assumes you mean broadcast to the network interface's broadcast address.
The above configuration should do the same for you as your socat but with additional configuration kplex also gives you the opportunity to do error checking, make the data also available on a tcp server, only forward certain sentence types etc.
Where I'm getting confused is the NTRIP client. This is not something I've had a lot of experience with but is it not usually the case that an NTRIP client will feed dgnss information to an associated gnss receiver or is this a program which synthesises a location by combining NTRIP data with an incoming NMEA datastream and makes it available over android's mock location provider? And was the role of the tcp server in your configuration to feed GPS *to* the tablet or to receive a stream of corrected GPS data *from* the client?
I recall from using navionics on my phone that it stops paying attention to internal GPS when you provide it with an external data feed.