Lake Pump Breaker Keeps Tripping

At my off grid cabin I have a submersible pump in the lake that feeds a pressure tank for water in the cabin.  The pump is on a dedicated 15A breaker in my panel.  Everytime the pump is called on it successfully fills the tank almost to end of cycle but then the breaker trips right before reaching final PSI (60).  Happens every time.  I have done much trouble shooting but can't solve it.  Any suggestions to figure this out would be much appreciated.

System:
- Red Lion RL12G05-2W1V
- 1/2 HP, 115V, 2-wire submersible
- Installed summer 2024
- Lake water system feeding pressure tank
- Pump left in lake over winter
- Franklin Electric disconnect/control box under cabin

Symptoms:
- Pump starts and pumps water normally
- Breaker hums while pump runs
- Pressure rises from ~50 PSI to ~58 PSI
- After ~20 seconds breaker trips before cutout pressure
- Voltage drops from 119.6V idle to ~111V under load

What I checked:
- Swapped breaker with known good breaker → same problem
- Pressure switch has proper voltage
- No obvious burned terminals
- Pump does move water before trip
- Lake had heavy algae/slime last summer
- Pump has never been pulled or cleaned since install

Wiring details:
- Pump label clearly says 2-wire 115V
- Lake cable has black/yellow/red/green conductors
In Franklin box:
- black → L1
- yellow → L2
- green → ground
- red unused/capped
Franklin box appears to be acting mainly as disconnect/junction, not true 3-wire control box

Questions:
Most likely cause?
overloaded/dirty pump?
failing motor?
bad underwater splice/cable?
voltage drop?
pressure switch issue?
Would algae/slime buildup on intake or impeller cause this exact symptom: runs ~20 sec then trips?
Is it common to leave lake pumps submerged year-round for multiple seasons?

Would your next step be:
-  pull/clean pump (maybe gunked up causing high current draw?)
- Trace the wires from lake back to Franklin box (huge pain in the butt to do this)
- bypass Franklin box,
- or replace pressure switch?

Thanks!!!

Hm. That's annoying.

Voltage drop -- how long is the wire run to the pump? What gauge wire is being used?

Douglas Alpenstock wrote:Hm. That's annoying.

Voltage drop -- how long is the wire run to the pump? What gauge wire is being used?

Yes! Very annoying.  The run is about 100 feet and the wire is 12 Gauge.  I think that is fine?

When the pump kicks in, I measured the voltage dropping to 111.4V while the pump was running, before the breaker tripped about 20 seconds later.