Many of you have probably already heard of this.
Primal Groudon's ability is Desolate Land, which prevents any changes to weather brought about by Hail, Sandstorm, etc. and also deactivates abilities such as Drizzle, Sand Stream, etc. In addition, water type moves are 100% ineffective under this "extremely harsh sunlight."
Primal Kyogre's ability is Primordial Sea, which also prevents any changes to weather, deactivates weather-inducing abilities, and render fire-type moves 100% ineffective under "heavy rain."
Both weather effects only last as long as either Groudon/Kyogre is actively in battle and it disappears when they are switched out.
So... Primal Groudon, whose Ground/Fire typing is 4x weak to water, now finds itself immune to water; whereas Primal Kyogre is also now immune to fire - a type that it only resists in the first place. Yeah, sounds fair.
They also have new signature moves: Precipice Blades and Origin Pulse.