[MENTION=81242]Lost Soul[/MENTION]
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5E is overbalanced towards martial characters and its getting old. Would you be happy being a martial character in a water adventure where missile weapons other than crossbows are useless and any non piercing melee weapon is heavily resisted or caused disadvantage on attack rolls? Should we have carried forward sneak attack immunities for broad ranges of monsters such as undead, golems and elementals? If you answers to these questions are a resounding yes than fine you can continue to argue with me that spell resistances and immunities should be in 5E. If not, then don't bother. Spells have been greatly nerfed in 5E in terms of both raw power and effect. They are not an old Basic or 1rst Edition feel by any stretch of the imagination.
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I would announce to the PC upfront that it is waterworld and what special rules might apply.
And TBH resstances and immunities are rare and weak in 5E compared to other Editions.