Two workers escaped with minor injuries after falling fell two-and-a-half metres in the incident last July.
Plymouth Magistrates’ Court heard that the workers were pouring concrete onto a deck to form the floor of a weights area at the dry dive centre.
But no temporary support was used and the combined weight of the two men and concrete caused it to give way.
An HSE investigation found that Balfour Beatty Construction Scottish and Southern Limited failed to ensure that sufficient steps were taken to protect workers during the construction work that led to the collapse.
The company did not follow its own temporary works procedure and should have checked that the deck was properly supported before allowing the work to commence.
Balfour Beatty Construction Scottish and Southern Ltd, of 24 Ravelston Terrace, Edinburgh pleaded guilty to a single breach of The Construction (Design and Management) Regulations and was fined £2,000 and ordered to pay costs of £5,347.
Speaking after the hearing, HSE Principal Inspector Andrew Kingscott said: “The two workers were fortunate not to have been seriously injured in this incident, which was entirely preventable had Balfour Beatty followed the proper temporary works procedure.
“The correct procedure would have highlighted the need to use a temporary support, such as stronger steel decking, propping or the construction of a temporary wall.”