The researchers designed, implemented and analyze a project to teach the contents of First Aid and Waves in English as a foreign language in Brazil using the Content and Language Integrated (CLIL) approach. Seventy participants belonging to two intact classes in a Navy Boarding School took part in the study. Both content and language teachers collaborated to plan and teach the contents. Data collection was carried out through questionnaires, class observation, and entries to teachers' diary. Overall results of the analysis of participants' perceptions on the approach used show that CLIL may be a relevant possibility in that context if certain aspects are considered carefully. Regarding the aspects related to the language of instruction, the study showed that CLIL should be implemented gradually and after students develop a certain level of proficiency in the foreign language. Regarding the aspects related to the content, it was found that participants' perception on CLIL in the First Aid class was much better than in the Waves class because the latter is perceived as being a more difficult content. This finding suggests that difficult contents might not be taught through CLIL depending on participants' language proficiency level. Finally, the study suggests that the implementation of CLIL poses many challenges for teacher practice and education that might be partially overcome through teacher collaboration.