Sometimes it's really worth to take a good rest.
If there is a continual overload for particular muscles you are simply risking too much in long term sustainability. There is a risk of microtrauma here and you can damage your body. One day rest now can save you another week of training when you are back in a good fit.
I don't know your workout but I experience tired triceps mostly when swimming crawl. I'd suggest you to switch to a backstroke at least for a while. You should swim the same distance by crawl and the same distance by backstroke to avoid disbalance. It can be in one workout day or in one week for example 3 days crawl and 3 days backstroke.
If it's not a problem you can go out of water for two days and try for example running or go to the gym and focus on your core (which will be used while swimming too!).
It has been already suggested in comments: if you can't be without water just do great leg workout for this time and let your arms rest.
I hope I don't need to mention proper warming up before training and stretching after it.
P.S.: You can try a massage too.