Whisk the mayonnaise, cream cheese, salt and pepper together in a large bowl then stir in the lemon juice until smooth.
Stir in the red onion and chives then stir in the tuna just until combined.
Cover and refrigerate for at least 30 minutes.
In the meantime, cut the ends off the cucumber then cut each one into thirds.
Slice each third in half lengthwise then scoop out the seeds using a spoon.
Turn them upside down on a paper towel to let any excess water drain out while the tuna chills.
Fill the cucumber halves with 2-3 tablespoons of the tuna filling and serve immediately with a sprinkle of freshly chopped chives for garnish if desired.
Notes
The cucumber boats should be served as soon as they are assembled but the filling can be stored separately in an airtight container in the fridge for up to 3 days.