First things I guess; is how bad ? are they just spots or was it a large puddle? what finish is on the wood, e.g. oil or lacquer? is it natural oak or stained, was it just clean water or did it have detergent or something else in?

Assuming just clean water on an oiled finish you may find that a gentle rub down with a fine abrasive paper and a new application of oil will solve the problem, - TEST A BIT FIRST. The grain may be raised hence the sandpaper but be careful and feather it out to the surrounding areas. If lots of spits and spots you may well be better incorporating the whole area with a light re-sand rather than each individual spot. Do this before resorting to wood bleachers like Oxalic acid.

I used to use oxalic acid to remove grey stains from the wooden frame of my Morris 1000 traveller. Has to be worth a try on an off-cut

Easiest form of oxalic acid though not pure is kettle descaler

Just goes to show you cannot trust anyone not to have an accident with something or another. I am surprised the decorator did not cover the floors first. When I had plasterers in I covered the floors with correx sheeting even though the floor was chipboard and was going to be later battened out for underfloor heating and engineered boards laid later. One has to think prevention all the time because some tradesmen don't know the meaning of the word
Good luck on solving problem.( I bought my boards unfinished so had control over repair materials. Used Osmo poly X oil great stuff). Hopefully you can get an answer on what the finish is, that has to be your best bet. I would not go near bleaching the spots, it is another can of worms involving recolouring the stain to match up. That is a very skilled job. If you draw a blank w.r.t doing a DIY fix there are furniture finishers about think there are franchises specialising in this sort of work much as there is on repairing chips and dents on cars. think furniture medic rings a bell. Charge the cost to the decorator.