Belgium flooding much of its country during world war one successfully slowed the German's enough to stop the Schlieffen Plan by preventing the French lines from being encircled for long enough for French and Common Wealth forces to redeploy to intercept the German line of defence. This this is a very effective method of areal denial and can be best used to disrupt massed armour and mechanised infantry advances as they are most limited by the routes they can take.
I believe that flooding certain areas to create a defense line can be used again today. Depending on the location and the type of opposition being faced, natural barriers can still be used successfully as defense in military tactics. This type of defense is affective against grown troops and some vehicles.
Yes, the Netherlands could use flooding as a military defense again in the future. Mankind has come a long way in military missiles and ballistics. However, in times of war, the only way to successful claim victory is to be standing on the enemy's land. Fortifying the land with water would mean that an additional physical barrier has been added. Netherlands would still need to continue update their defenses for missiles and ballistic protection as long range weaponry could cross the floodplain easily.
I think that the technique of flooding could absolutely prove an effective military tactic. Humans are incredibly susceptible to natural occurrences and environmental hazards that are beyond our control. Floods are very hard to predict in terms of size and level of danger, so using them as a method of defense could be very beneficial.
No, flooding should not be used to create a military defensive line. This may have been a successful tactic in the past, before the use of aircraft in warfare. However, since the use of planes and other aircraft is now a common form of defense, flooding would ultimately be a waste of time, because enemy planes could simply fly over the flooded area and attack.