Am I using these prepositions correctly?

Hi,

please let me know whether I am using the prepositions in the following sentence correctly or not. Let me know the right possible sutitutions.

1. He shows a lack of enthusiasm in his work.
2. We must get in contact with Mary.
3. The police have their suspicions for him. No, eg about or of
4. There was an investigation about the fire. Say of or into
5. He is walking beside the sea.

Clive