It sounds like it's being starved on fuel. You can split the possibilities in half by feeding the motor from a hose in a can or bottle of gas. If it runs OK like that, you have a fuel delivery problem, clogged filter, collapsing fuel hose or squeeze bulb. If not, then you can start looking at the motor, internal fuel filter, dirty carbs, etc.
2015 Seaark 1872 MV CC Etec-90
Two Helix 12 CHIRP SI's, , MinnKota Riptide ST80/i-pilot Link, Bob's Hydraulic
Bitter Gun Owner