'ins Wasser' is the same as 'in das Wasser'. 'Gehen im' would mean that the flowers are in the water and walking there. So, grammar is fine. But none of this makes any sense...
Thank you Wataya, just tu sum up Can you use "in das" instead of "ins" if you like? or is it mandatory to unite them
The German sentence doesn't make much sense. (It's grammatically correct, but it literally means 'flowers walk into the water'). Maybe they meant 'Blumen gehören ins Wasser' or 'Blumen kommen ins Wasser'. No idea.