Swimming in the ocean is a very exciting activity. Here 'swimming' is a -

• Swimming in the ocean is a very exciting activity. Here 'swimming' is a - non -finite verb.

• Non-finite verb: 
- A non-finite verb is a verb form that does not show tense.
- Most non-finite verbs found in English are infinitives, participles, and gerunds.
- যে verb দ্বারা sentence এর বক্তব্য শেষ হয়না ও কোন Subject এর person ও number দ্বারা যার রূপ নির্ণয় করা যায় না তাকে Non finite verb বলে। 

• Swimming in the ocean is a very exciting activity.
- এই sentence এ 'swimming' হচ্ছে subject হিসেবে Gerund.
- তাই, এটি Non-finite verb.
- এটি দ্বারা কোনো action বুঝাচ্ছে না।

• Finite verb: 
- A finite verb is a verb that has a subject and shows tense.
- Finite verbs are not gerunds, infinitives or participles, which are classified as non-finite verbs.
- Tense এর গঠন অনুসারে যে verb বসে, সেটিই Finite Verb.

Source: Cambridge Dictionary.