Polaris Genesis watercraft forum - polaris genesis jet ski

my genesis won't run at low rpms, stalls out...seem to be running right at faster speed

I recently purchased two '99'genesis wave runners for a reasonable price due to the motors being seized. After further inspection i found that there was no scorching in the jugs or pistons, but found piles of sand in the water jacket. what steps should i take to make sure i flush the system completely? Also i'm not to familiar to the cooling system on wave runners, is it possible the water pump is driven from the crank and is keeping the motor from rolling over due to the sand?