No water out telltale in gear

[flash drive]

8 hp Mercury that I replaced the water pump on. Started it up nice flow from telltale at idle in neutral. Let it run a few minutes then took it out for a spin. In reverse there was still flow but not as strong as it was. In forward the flow was sputtering and when I throttled up a little no more flow just puffs of steam. Brought back to idle/ neutral and flow came back. Pulled lower unit again to check everything and all looks good. Any ideas? Obviously no hose blockage.