Learn Spanish (Latin America): Debe de ser agua, porque hay un charco en el suelo. - It must be water, because there is a puddle on the ground.

The phrase deber de followed by an infinitive like ser is used to express a strong guess or probability, meaning "it must be". Grammatically, without the de, debe ser means an obligation ("it has to be"). However, in everyday spoken Spanish, native speakers often drop the de and use debe ser for both probability and obligation.

Why isn't there an article before "agua", such as "el agua"?

Just like in English, we do not use an article when referring to an unspecified amount of an uncountable substance. You are simply stating "it is water", not "it is the water".

Could I use "piso" instead of "suelo" here?

Yes! In Latin America, piso is very commonly used for the floor inside a building, while suelo can mean the ground outside, the earth, or the floor in a general sense. Depending on whether this puddle is indoors or outdoors, either word works perfectly.

Why is "porque" written as one word without an accent mark?

When written as a single word without an accent, porque means "because". It connects your guess about the water to the evidence of the puddle. If you were asking a question to say "why", it would be written as two words with an accent: por qué.