I have had trouble with the engines starting during cranking.
525SC's
new batteries
use both batteries always

when I am cranking it does not start, I can carry the momentum and when i release the start mode on the key they start.
I replaced one of the key swiches but the issue did not resolve.

any advice on trouble shooting, is this common, ECM seeing a voltage drop?

Ramb

I repaired this on on a handful of boats over the years. Issue with All of them (4 or 5?)had bypassed the remote solenoid for whatever non thinking reason...therefore they ran the wire right from ign/switch/shifter directly to the solenoid on the starter instead. 1 had just come from a marina whom had replaced the starter.

wired this way, there wan't enough electricity (while starter cranking) to fire the ignition,until a millisecond after cranking stopped.

these where all carb'd application too, yours maybe a different part/rigging cause with same issue, but chances are voltage drop will be the cause.

thanks SB

With new batteries I wonder if my starter cables should be updated/doubled up. everything is clean and fresh water use, but takes a lot to crank blower motors.

So I will put a volt meter on and measure voltage drop during crank.

Can the ECU's become more sensitive to ignition voltage? or a coil issue that can be updated?

Ramb

It also could be a bad key switch. when you turn the key it may not be sending power to the run terminal untill after you release the key from the start position.
check it out with a volt meter. I have had this before.
It could also be a weak kill switch or neutral start switch.
They are two separate circuits.
You can engage the starter but the engine won't start unless the run circuit is energized.

Batteries don't warrant new cables. Bad cables warrant new cables.

Do you know what the remote solenoids are ? And what they do ?

525SC should fire within 1-3 seconds of cranking. Two pumps of the throttle and turn the key.
Measure your voltage at a purple/ignition on wire while cranking.

MAke sure connections on the round plug are clean, sounds like you are missing the voltage while cranking, check continuity from switch to the coil on I believe the purple wire, you can look up diagram to trace wiring and make sure what color wire gets voltage during crank, need crank and start voltage at the same time to fire it.