Carlyb, it is that heart in throat moment that stays with you! Of course you have had a huge fright and will probably remember this longer than your ds does!
I speak from experience, last summer ds twins (then aged 18 months) were being bathed by DH. He stupidly went into their bedroom for something. I was tidying up downstairs when I heard this noise, I could feel inside it was something awful. When I raced upstairs ds twin1 was underwater, eyes wide with fright. Luckily he had held his breath so all we had to deal with was the terror of it all.
Needless to say DH was yelled at, screamed at and ignored for a week I was so mad! Within minutes of it happening ds twin1 was wanting back into the bath to play with his brother! I still get chills thinking about it and, believe me, DH never leaves them!
You are not stupid. It is why these things are called accidents. I sometimes think we are given extra guilt with all the info we are given when pregnant etc. The emphasis is on, here are how accidents happen, if you let them happen knowing this you are an idiot. No, accidents sometimes happen no matter what.
Just to confirm how bad as parents we are ds twin2, not to be outdone, fell down the stairs that summer as well! Despite the stair gates. (He was unharmed and the hospital doctor said - to comfort me I think - not to worry, babies bounce!) Waaaa!