The important thing to remember when designing enemies is that Form follows Function. This applies to any part of their design - their size, their behaviour, their speed, movement, attacks, aggression - their form must be the inspiring force when it comes to their design.
Piranha
The piranha enemy was designed to force/chase the player towards the river bank so that it is within the illegal logger's firing range. One of its attacks is a tunneling blow to the underside of your raft, making you more prone to falling over and ending your delivery run. Another thing it can do is jump out of the water and slap you with its scaly fins, which deal 10% of your heatlh in damage.
When defeated by 3 well-timed blows of your paddle, you will receive its tooth, an ornament for your spear that will make it faster to kill other piranhas with.
Illegal Logger
The Illegal Logger shirks away by the side of the river, waiting to strike a final blow with its stones or bullets. They fear your deliveries and aid may eventually help the fishing tribes get rid of them - an ultimately true statement if you manage to deliver your goods to them. They coordinate with other loggers to time their gunshots everywhere to be untraceable. These attacks are very deadly, with the bullets dealing 40% of your health in damage, and the stones they throw dealing only 10%. These also make you stumble and fall off your raft if you're standing by the edge of it, similar to the piranhas that they coordinate with.