Just 2 fun little ideas to make the water a tad bit less safe (in the jungles at least).
Piranha
The piranha would be a type of fish slightly larger than the other fish in the mod so far. Since it is the most iconic, and therefore most likely to be recognized and avoided by early players, the texture would be that of a fire-belly piranha. These are traditionally indigenous to the fresh-water rivers of south america, and would therefore would spawn in the rivers of the jungle/mangrove. They might also spawn in mud-bottomed still-water, but I'm not sure on that one.
AI
Piranhas spawn in packs, and will try to remain in packs if possible. When a source of meat enters their water they will attack it mercilessly until it dies, at which point each piranha will consume a single piece of meat sating them for about 10 minutes. They would also periodically attack other fish (not piranhas) for meat. In addition they will band together to protect their own, and will attack the player similar to zombie pigmen when one is struck.
Sound
When hunting or being hurt, the piranha would use the wolf's aggressive bark sound, for other sounds it would use the normal fish sounds.
Attack
Their attack would deal 2hp of damage, and would inflict 2 blocks or so of self-knockback simulating getting out of the way to either allow others to attack, or to avoid retaliation.
Electric Eel
The electric eel would be a multisegmented (probably 5-7) dark brown/green creature. It is indigenous to calm fresh water of south america such as swamps or slow-moving rivers. This would be best represented by a moderately low spawn rate in swamp biomes and non-river-biome water sources above mud blocks in the jungle, as well as a very low spawn rate in rivers in the jungle and mangrove.
AI
It would generally be passive, but when attacked, or if threatened by proximity to the player, it will release a shock in an attempt to save itself. Then flee. Additionally, it will rarely shock then attack other fish, eating them.
Attacks
Shock
The eel's shock attack is AoE with a radius of 3 blocks in water, and 0 blocks (requires direct contact) out of it. It would drop non-aquatic entities by 2 air bubbles, would deal 2hp worth of AP damage, and would turn the screen of any players bright blue which will fade over 2 seconds. Any entities (including player), would gain the "stunned" or "paralyzed" debuff for 1 second, locking all movement. Upon use this will release a static shock sound effect. After using shock it will have a 10-second cooldown during which it will flee from threats, or use its bite attack on prey, or on threats if it proves unable to flee them.
Bite
It would also have a secondary bite attack which would deal 1hp of damage. ...and I already detailed use of it above, why do I even have a separate section for this?
Well, yeah. There you go, a "short" suggestion to make the jungle a tad bit more exciting.