They are intended to be watertight, but you need to take some precautions.
In this Instructable page on MC4 Solar Connectors you see red rubber sealing rings in the picture from step 5:
The Wikipedia page on MC4 connectors mentions:
For a proper seal, MC4s require the usage of a cable with the correct diameter
Connection is made by use of a special crimping tool, alternatively by soldering.
This REC Guide to Best Practice suggests what (not) to do:
Ensure sufficient air circulation around connectors.
Keeping humidity away from electronic connections is also important. Solar panel connectors must not be left to stand in permanent or regularly occurring pools of water. Likewise, after rainfall, connectors should be given the opportunity to dry completely. Although the connectors used by REC are rated to IP67, moisture can build up between the mounting structure and connector.Fig. 9 shows a connector secured tight to the mounting structure. While the panel above protects the connection from rain; humid air, mist or condensation can cause moisture to accumulate between the connector and the mounting rail. This layer of moisture is caught between the connector and rail and can slowly work into the connector through capillary action. Here it can erode the metallic connection.
From that guide:
Fig. 9: Fastening of the connector tight to the mounting structure can cause moisture build up between connector and rail, damaging the connection