Dehydration, sun and bugs are common enemies in Belize, but there are easy things you can do to combat them.  

Dehydration - Drink plenty of water. Drink water before you even feel thirsty. Also, consider bringing electrolyte tablets or a drink with electrolytes as a backup measure.

Sun - Sunscreen is partially effective, and, unless it says it's "Reef Safe", then it is harmful to the coral reef.  So, remember your hat, and consider wearing a long sleeve shirt and long pants, or wearing a rash guard over your swimsuit.

Bugs - Bugs love the Jungle. Repellents work well for some bugs and poorly for others. Consider wearing a long sleeve shirt and long pants when in the jungle. On the beach you may encounter sand flies. All you need to combat them is a thin layer of something oily. Coconut oil is a favorite, but baby oil, Skin So Soft, and basically any other oil works too.

Temperatures don't really vary that much. You can generally expect lows in the 70s and highs in the mid to high 80s. However, in the winters it might get down into the 60s, and occasionally during the summer it might get up into the 90s.

Also, it is generally humid, and there is usually an ocean breeze. This can make it feel cooler at night than the temperature would indicate, so bring a light jacket for the evenings.

There is a so called "rainy season", and not-so-rainy season.  The rainiest season is from July to October.  This would be approximately twice as rainy as the "dry season".  But, it generally doesn't rain all day in Hopkins...usually the rain is at night or in brief outbursts.