@UmmH
My point, Bluntness, is that in a private space and particularly in a hot tub someone could have been naked, so not appropriate for a landlord to just let himself into the garden unannounced. HE didn't know who was going to be there or what state of undress they might have been in. By the time he realised it would have been too late.
Well said. I can't believe some of the minimising on here over peoples genuine feelings.
Feeling vulnerable and awkward and embarrassed and having people scoff and mock like it's nothing. 
No wonder some women are afraid to report anything or say anything (when they feel violated or vulnerable or worried or threatened in any way!)
Just when you think we're moving on in leaps and bounds as a society, you realise we're not, and some people are stuck in the dark ages.
And @TotinEggs although saying YABU is not minimising peoples feelings and worries, some of the comments people have come out with IS minimising peoples feelings and worries.
@OxfordCat
Such a load of bollocks on this thread! Ignore pp trying to minimise your feelings OP. If in your gut it felt wrong / somewhat rude, or even slightly pervy, you are entitled to feel that way!
To those snotty pp saying "it's just like being on the beach" - no it isn't! The OP and her ds were not in a public place, they were in the back garden, quite reasonably expecting privacy.
To those saying "he's doing his job and probably thought you'd be out" - yes but once he entered the garden and saw that they were in the tub he would then have realised they were NOT out! And at that point a reasonable and polite person would have offered to come back later. Even better, as has been suggested, he could have a set time each day so guests are warned in advance.
All of this. Well said!
I bet the owner would not have come along to 'check the hot tub' if there had been 2 burly, hairy-arsed MALE rugby players in at the house. Two young 20-something females though??? Hmmmmm ....... yeah' let's just pop along and have a random impromptu visit with no warning eh?' 