The Water Elemental(Basic Rules, 158) stat blocks states with my empahsis:
Each creature in the elemental’s space must make a DC 15 Strength saving throw. On a failure, a target takes 13 (2d8 + 4) bludgeoning damage. If it is Large or smaller, it is also grappled (escape DC 14). Until this grapple ends, the target is restrained and unable to breathe unless it can breathe water. If the saving throw is successful, the target is pushed out of the elemental’s space.
The elemental can grapple one Large creature or up to two Medium or smaller creatures at one time. At the start of each of the elemental’s turns, each target grappled by it takes 13 (2d8 + 4) bludgeoning damage. A creature within 5 feet of the elemental can pull a creature or object out of it by taking an action to make a DC 14 Strength check and succeeding.
There is no option for grappling a Huge, just that if it is large or smaller it is grappled. Huge and Gargantuan creatures are not valid targets for this grappling effect and subsequent damage. And the 2nd paragraph of the Whelm action still contains the requirements on size.
Grappling outside of Whelm
While there are rules around grappling creatures one size larger, those rules don’t apply to Whelm’s actions. Only the requirements within the Whelm action are relevant to determining how the Whelm works.
Meaning that even if they could grapple a Huge creature (which may be up to a DM to determine), they can not Whelm it.