Multi-causal rough idle and no power, Yaris 2004

[Dan789]

Hello all. Grateful for any advice here. First I should point out I’m not mechanically minded at all, the last time I repaired a car was in the late 90s when I changed a fuel pump on a 1979 polo!
I have Yaris Verso 1.3 petrol auto 2004.
Recently we got rough and unsteady low idle, occasional loss of power. Last weekend I cleaned the throttle body and the MAF sensor, couldn’t check plugs as did not have the correct size tool.
Thursday I noticed water sitting on top of the engine block around the top of the coil packs. I was going to drive it to get rid of it before taking off coil packs, but had zero power in gear. Could rev fine in neutral.
Today I removed plug in leftmost cylinder (1?) and there’s oil on the plug, plus the middle, top-side, looked a bit rusty. Looking into the plug well it looks like oil, plus it looks kind of grubby. All other plug wells are clean.
I tightened the bolts on the gasket cover (? The top of the engine block, I thought they might be loose causing oil leak into plug well).
I cleaned the spark plug and wiped the end of the coil pack, refitted and started. Idle fine, power restored when driving. I’ve only driven in the driveway, as I have no idea how long it will be ok for.

So, my questions are: what’s with the water sitting on top of engine block and could some of it have seeped into the plug well? If so how should I clean it all out?

For the oil on plug should I wait to see if tightening the bolts fixes it or do I need a new gasket seal?

Long post I know, trying to get as much detail as possible in.

[ex-x-fire]

The water was getting on the coil boots causing the spark to jump to ground instead of firing the spark plug. Figure out why the engine keeps getting wet up there.

[Dan789]

OK great, thank you.