Some time ago, I recall seeing pictures of planters that had PVC tubes placed in the drainage holes and that might be an idea where the tubes are cut long enough (and/or use elbow pieces) to direct the runoff away from the deck. This assumes using a planter with a bottom (lined) with the drainage tubes going through holes in the deck. Alternately, you could try planters with a false bottom that serves as a reservoir (with an overflow opening). That way, no water drains onto the deck unless there is a large amount of excess that would come out of the overflow hole. I think in that case, it might be better to have a non-wood reservoir (if the planter is wood and built by hand, perhaps put a plastic catch basin inside the false bottom). You would need to make sure that the planter + soil + water doesn't exceed the deck's weight-bearing load limits in any case.