Most players' intuition would be that it doesn't trigger. In this case the intuition is correct (rare for this blog.) The interesting question is, however, why exactly.
Declaring attackers is a bit more complex of an operation than most players are aware of. It actually resembles the process of casting spells and activating abilities.
Casting a spell technically follows the following procedure (highly condensed here): Announce the spell you are going to cast by revealing it (it moves to the stack), make any possible choices for the spell (possible modes, value of X, alternative or additional costs to be paid, color choice of hybrid mana symbols), choose targets (if it has any), determine the total cost of the spell, activate mana abilities, and finally pay the costs. After all this has been successfully and legally done, the spell is considered successfully cast (and anything that triggers on such a spell being cast triggers now.)
(If at any point it turns out that one of those actions is illegal, everything above is reversed, and none of it is considered having happened.)
Activating abilities is almost identical to the above (with the only difference being that it's not a spell that's put on the stack, but an ability.)
Perhaps a bit surprisingly, but very consistently, declaring attackers (and also declaring blockers) follows a procedure that's very similar. First you announce which creatures are going to attack, then you choose what they are attacking (ie. which opponents or planeswalkers those opponents control), then you determine the possible costs required for this to happen, then you can activate mana abilities, and finally pay the costs (if there are any). After all this has been successfully and legally done, the creatures are considered successfully declared as attackers, and anything that triggers on such a creature attacking triggers now.
(If at any point it turns out that one of those actions is illegal, everything above is reversed, and none of it is considered having happened. This includes in this case checking that no restrictions are broken, and that the amount of requirements that can be fulfilled with a legal declaration has been maximized.)
Note how costs are paid before a creature is considered having attacked, and returning an enchantment to its owner's hand is a cost for Floodtide Serpent to attack. Thus the ordeal returns to your hand before the serpent has attacked, and thus it doesn't trigger.
